What does “заступиться” mean in Russian?

Заступиться за кого-то means to step in to defend someone — to take their side when they’re in trouble. The word is built from за- (the prefix of “in defense of / behind”) + ступить (to step) + -ся (reflexive). Literally: to step in behind someone as a shield. The cat in the clip does exactly that — it sees its owner wrestling a guest and immediately decides to заступиться за хозяина, biting everyone in range.

The imperfective pair is заступаться (ongoing habit) versus the perfective заступиться (a single decisive act): он всегда заступается за друзей (“he always stands up for his friends”) but она заступилась за него вчера (“she stood up for him yesterday”). The preposition за is fixed — you always заступиться за кого-то (accusative). Related forms worth knowing: заступник (defender, champion) and заступница (its feminine equivalent), used in both everyday speech and older religious contexts where saints intercede on behalf of believers.