Before, guns had to have a round put in the chamber through operating a bolt. when the first self loading guns were made, they were referred to as “automatic” cause you didnt have to do anything to get the next round into the chamber. (This is from memory and could be wrong or off)

Today automatic colloquially means the firearm repeatedly fires as long as the trigger is held, though in pistols the phrase automatic seems to have held on to its meaning as self loading.