There is no steady wisdom for one who is not disciplined, and no contemplative focus for one who is not disciplined. Without that focus there is no peace, and for one without peace, where could happiness come from?
Explanation
Krishna is explaining to Arjuna why mastery of the senses matters. One who is not yoked (ayukta), meaning not disciplined and centered on the Self, lacks steady understanding (buddhi) and sustained contemplation (bhāvanā). Without these there is no peace (śānti), and so no happiness (sukha). Commentators differ on bhāvanā: some take it as meditation on the Self, others as steady devotion or inner resolve.