Other forms: slots
A slot is a narrow opening made for something to be dropped or slid into. A coin, a letter, a card.
It is always deliberate, which separates a slot from a crack or a gap. Machines have them, which is where a slot machine comes from, and so do letterboxes and card readers. The figurative sense is about time, so a slot in the schedule and a prime-time slot are both standard. As a verb, slotting something in means fitting it into a gap, in a machine or a diary.
A slot is a lock. Middle Dutch slot meant a bolt or a lock, from a root meaning to shut, and German still uses Schloss for a door-bolt. The narrow opening is what the bolt slid through.
Old Norse
slóð — track
Old French
esclot
Middle English
slot
English, today
slot
From Middle English slot, from Old French esclot, likely from Old Norse slóð (“track”). As a gambling machine, via clipping of slot machine. Compare sleuth. The scheduling (calendar) sense is by a metaphor whereby the time span is equated with the segment of a page or part of a device that represents it.
Etymology from Wiktionary, CC BY-SA