When we knock under, we surrender or accept defeat. However, there are many synonyms for this phrase that are more apt or dramatic depending on the context. For instance, one might capitulate, give in, yield, submit, concede, cave in, surrender, or wave the white flag. Alternatively, one might quit, back down, buckle under, fall to one's knees, raise the white flag, succumb, or bend the knee. These terms all imply different degrees of surrender and could be used to evoke different emotions or tones in writing or speech. Whatever word you choose, the most important thing when you knock under is to remember to take care of yourself, rest, and refuel before continuing the fight.