A required wait before a thing may be done again. A cooldown protects what repetition would exhaust.
English compound of cool (Old English col, not warm) + down (Old English dune, descending), formed in the 20th century for a period in which something is deliberately let ease off; a required wait before a thing may be repeated.
The compound is plain and recent: a thing let cool, then let settle lower, before it is asked to work again. What it guards against is never the next use itself, only a use arriving before the last one has finished spending itself.