Synonyms:

atha–therefore; cet–if; tvam–you; imam–this; dharmyam–religious duty; sangramam–fighting; na–do not; karisyasi–perform; tatah–then; sva-dharmam–your religious duty; kirtim–reputation; ca–also; hitva–losing; papam–sinful reaction; avapsyasi–do gain.

Translation:

If, however, you do not fight this religious war, then you will certainly incur sins for neglecting your duties and thus lose your reputation as a fighter.