The "slot machine" term derives from the slots on the machine for inserting and retrieving coins. "Fruit machine" comes from the traditional fruit

on the spinning reels such as lemons and cherries.

A slot is a narrow opening in a machine or container, for example a hole that you put coins in to make a machine work. He dropped a coin into the slot and dialled the number. If you slot something into something else, or if it slots into it, you put it into a space where it fits. He was slotting a CD into a CD player.