means A narrow opening or designated space into which something fits — whether a coin into a machine, an appointment into a schedule, or a card into a reader.

from From Middle English 'slot,' meaning a hollow or depression, especially the one at the base of the throat — likely from Old French 'esclot.' The 'narrow opening for coins' sense is a much later, industrial development, and the 'allotted time or position' sense (a slot in the schedule) came later still, by metaphor. The gambling 'slot machine' lent the word its hungry reputation in the modern era.