Sanskrit
सद्भावे साधुभावे च सदित्येतत्प्रयुज्यते । प्रशस्ते कर्मणि तथा सच्छब्दः पार्थ युज्यते ॥
sadbhāve sādhubhāve ca sad ity etat prayujyate praśaste karmaṇi tathā sacchabdaḥ pārtha yujyate
Translation
"Sat" is used in the sense of reality and goodness; likewise, O Pârtha, the word "Sat" is used in the sense of a good work.
Annie Besant · The Bhagavad-Gita, translated by Annie Besant (4th ed., 1922) — Wikisource · Public Domain
Explanation
Sat is doing double duty as a word — it names both what is metaphysically real (sad-bhāva, existence itself) and what is ethically good (sādhu-bhāva, virtuous action), and this verse says the two senses aren’t separate meanings so much as one continuous idea: genuine being and genuine goodness are the same kind of “real.” That collapsing of ontology and ethics into a single word is a distinctly Vedic move, one with no clean English equivalent, which is part of why the translator leaves the term untranslated.