We don't have synonyms for side arm yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are sideward, sidearm and sidebar.
(n) :
Alternative spelling of sidearm. [A personal weapon, either a firearm (handgun type) or a cold weapon (such as a sword), carried on the hip in a belt, sheath, holster, etc., for rapid access.]
(n) :
Alternative spelling of sidearm. [A personal weapon, either a firearm (handgun type) or a cold weapon (such as a sword), carried on the hip in a belt, sheath, holster, etc., for rapid access.]