B.G 2.66 and 67

नास्ति बुदि्धरयुक्तस्य न चायुक्तस्य भावना। न चाभावयतः शान्तिरशान्तस्य कुतः सुखम्॥६६॥

nāsti buddhiḥ ayuktasya na ca ayuktasya bhāvanā। na ca abhāvayataḥ śāntiḥ aśāntasya kutaḥ sukham॥66॥

[नास्ति (nāsti) - there is not; बुदि्धः (buddhiḥ) - wisdom; अयुक्तस्य (ayuktasya) - of the unsteady; न (na) - not; च (ca) - and; अयुक्तस्य (ayuktasya) - of the unsteady; भावना (bhāvanā) - contemplation; न (na) - not; च (ca) - and; अभावयतः (abhāvayataḥ) - of one without contemplation; शान्तिः (śāntiḥ) - peace; अशान्तस्य (aśāntasya) - of the peaceless; कुतः (kutaḥ) - whence; सुखम् (sukham) - happiness;]

(The unsteady person has no wisdom, nor any contemplation; without contemplation, there is no peace, and without peace, how can there be happiness?)

इंद्रियाणां हि चरतां यन्मनोनुविधीयते। तदस्य हरति प्रज्ञां वायुर्नावमिवाम्भसि ॥६७॥

indriyāṇāṃ hi caratāṃ yanmano'nuvidhīyate। tadasya harati prajñāṃ vāyurnāvamivāmbhasi ॥67॥

[इंद्रियाणां (indriyāṇām) - of the senses; हि (hi) - indeed; चरतां (caratām) - moving; यत् (yat) - which; मनः (manaḥ) - mind; अनुविधीयते (anuvidhīyate) - follows; तत् (tat) - that; अस्य (asya) - his; हरति (harati) - carries away; प्रज्ञाम् (prajñām) - wisdom; वायुः (vāyuḥ) - wind; नावम् (nāvam) - boat; इव (iva) - like; अम्भसि (ambhasi) - on the water;]

(For, just as a boat on the water is carried away by the wind, so too, the wisdom of a person whose mind follows the roaming senses is carried away.)

For one who is without the gracefully engaged mind, there is no wisdom. Without the graceful attitude, one does not get liberated. Without liberation, where is happiness? Such a mind (without grace) is sub-servient to the movement of senses. Wisdom is stolen from such a mind the way the windy waters carry away a ship.

Gīta Bhāshya 2.66 and 67

Restraining the mind faculty is not suitable for the ungraceful mind. 'anu vidhīyatē'- lord instruments as per one's senses. 'harati prajñāṁ' - arising of wisdom from knowledge is prevented and even the existing wisdom shrinks.

प्रसादाभावे दोषमाह उत्तराभ्यां श्लोकाभ्याम्। न हि प्रसादाभावे युक्तिः चित्तनिरोधः। अयुक्तस्य च बुद्धिः सम्यग्ज्ञानं नास्ति। तदेव उपपादयति न चायुक्तस्येति। शान्तिः मुक्तिः।

prasādābhāve doṣamāha uttarābhyāṃ ślokābhyām। na hi prasādābhāve yuktiḥ cittanirodhaḥ। ayuktasya ca buddhiḥ samyagjñānaṃ nāsti। tadeva upapādayati na cāyuktasyeti। śāntiḥ muktiḥ।

[प्रसाद-अभावे (prasāda-abhāve) - in the absence of clarity; दोषम् (doṣam) - fault; आह (āha) - states; उत्तराभ्यां (uttarābhyām) - by the following two; श्लोकाभ्याम् (ślokābhyām) - verses; न (na) - not; हि (hi) - indeed; प्रसाद-अभावे (prasāda-abhāve) - in the absence of clarity; युक्तिः (yuktiḥ) - reasoning; चित्त-निरोधः (citta-nirodhaḥ) - control of the mind; अयुक्तस्य (ayuktasya) - of the unrestrained; च (ca) - and; बुद्धिः (buddhiḥ) - intellect; सम्यक्-ज्ञानम् (samyak-jñānam) - right knowledge; नास्ति (nāsti) - does not exist; तत्-एव (tat-eva) - that itself; उपपादयति (upapādayati) - establishes; न (na) - not; च (ca) - and; अयुक्तस्य (ayuktasya) - of the unrestrained; इति (iti) - thus; शान्तिः (śāntiḥ) - peace; मुक्तिः (muktiḥ) - liberation;]

(The fault arising from lack of clarity is explained in the next two verses. Without clarity, there can be no reasoning or control of the mind. For one who is unrestrained, there is neither intellect nor true knowledge. This is established: for the unrestrained, there is neither peace nor liberation.)

Defect resulting from not having a serene mind is described in the next two verses. 'na yukta' means nongraceful mind, and because of which it cannot restrain the mind faculty. With such an unsuitable intellect, it is not possible to get perfected knowledge. Therefore, it is ascertained 'na cāyukta', i.e. 'it is not suitable' in the verse.
'shānti' means liberation.

"शान्तिर्मोक्षोथ निर्वाणम्।"

"śāntir mokṣotha nirvāṇam।"

[शान्तिः (śāntiḥ) - peace; मोक्षः (mokṣaḥ) - liberation; उथ (utha) - or; निर्वाणम् (nirvāṇam) - nirvana;]

(Peace, liberation, or nirvana.)

'Shanti', 'Mōksha' and 'Nirvāṇa" are synonymous,

इत्यभिधानात् ॥६६॥

ityabhidhānāt ॥66॥

[इति (iti) - thus; अभिधानात् (abhidhānāt) - from the statement;]

states the dictionary.

कथं अयुक्तस्य भावना न भवति। आह - इंद्रियाणामिति। अनु विधीयते क्रियते ननु ईश्वरेण इद्रियाणाम् अनु। "बुदि्धर्ज्ञानम्'' इति वक्ष्यमाणत्वात् । प्रज्ञां ज्ञानम् उत्पत्स्यदपि निवारयतीत्यर्थः। उत्पन्नस्यापि अभिभवो भवति ॥६७॥

kathaṁ ayuktasya bhāvanā na bhavati। āha - indriyāṇāmiti। anu vidhīyate kriyate nanu īśvareṇa idriyāṇām anu। "bud'dhirjñānam'' iti vakṣyamāṇatvāt। prajñāṁ jñānam utpatsyadapi nivārayatītyarthaḥ। utpannasyāpi abhibhavo bhavati॥67॥

[कथं (kathaṁ) - how; अयुक्तस्य (ayuktasya) - of the unsteady; भावना (bhāvanā) - contemplation; न (na) - not; भवति (bhavati) - arises; आह (āha) - he said; इंद्रियाणाम् (indriyāṇām) - of the senses; इति (iti) - thus; अनु (anu) - after; विधीयते (vidhīyate) - is regulated; क्रियते (kriyate) - is done; ननु (nanu) - indeed; ईश्वरेण (īśvareṇa) - by the Lord; इद्रियाणाम् (idriyāṇām) - of the senses; अनु (anu) - after; "बुदि्धर्ज्ञानम्'' ("bud'dhirjñānam'') - "intellect and knowledge;" इति (iti) - thus; वक्ष्यमाणत्वात् (vakṣyamāṇatvāt) - because it will be said; प्रज्ञां (prajñāṁ) - wisdom; ज्ञानम् (jñānam) - knowledge; उत्पत्स्यदपि (utpatsyadapi) - even if it arises; निवारयती (nivārayatī) - prevents; इत्यर्थः (ityarthaḥ) - this is the meaning; उत्पन्नस्यापि (utpannasyāpi) - even of what has arisen; अभिभवो (abhibhavo) - overpowering; भवति (bhavati) - happens;]

(How does contemplation not arise in one who is unsteady? It is said, 'of the senses.' Regulation and action follow the senses, indeed by the Lord. "Intellect and knowledge," as will be explained. Wisdom and knowledge, even if they arise, are prevented—this is the meaning. Even what has arisen is overpowered.)

Why one without graceful attitude does not obtain (serenity)? This question is answered by 'iṁdriyāṇām' verse.
'anu vidhīyatē' means lord instruments as per one's senses. Therefore, it is said "Wisdom, knowledge and other faculties of living beings are controlled by Me" in 10.4 verse.
'harati prajñāṁ' means arising of wisdom from knowledge is prevented and even the already produced wisdom shrinks.

Gīta Tātparya 2.66 and 67

'shānti' - also means honest endeavour towards the lord.

शान्तिः भगवन्निष्ठा।

śāntiḥ bhagavanniṣṭhā।

[शान्तिः (śāntiḥ) - peace; भगवन् (bhagavan) - O Lord; निष्ठा (niṣṭhā) - steadfastness;]

(O Lord, may there be peace and steadfastness.)

'śāntiḥ' means honest endeavour towards the lord.

"शमो मन्निष्ठता।"

"śamo manniṣṭhatā।"

[शमः (śamaḥ) - tranquility; मन्निष्ठता (man-niṣṭhatā) - steadfastness in Me;]

(Tranquility and steadfastness in Me.)

"The root 'sha' means sincerity of the mind"

इति हि भागवते ॥६४-६६॥

iti hi bhāgavate ॥64-66॥

[इति (iti) - thus; हि (hi) - indeed; भागवते (bhāgavate) - in the Bhāgavata;]

-so stated in Bhagavata.