In English, I guess "arms" means "weapon" wheareas 'an arm' means a body part. It is one of those nouns that change meanings according to its number. i.e. 'A color' means a color ; blue, red, white whereas 'colors' means 'a flag'.
To arms! is an idiom that was shouted out loud when urging soldiers to prepare for the battle ahead in medieval times meaning "ready your weapon!" may be go "to" the weapon and grab them and ready them for the battle ahead.