V. S. Apte, The Practical Sanskrit–English Dictionary. Look up any word of the Gita in Devanagari, IAST or Harvard-Kyoto, or browse the headwords letter by letter.
Entry तर्हि
तर्हिtarhiind.At that time, then.
[तद्-र्हिल्]
At that time, then.
In that case; यदा-तर्हि 'when-then'; यदि-तर्हि 'if-then'; कथं तर्हि 'how then'.