let up vs letup

let up is not a noun while letup is a noun.

Word	Noun	Adjective	Verb	Adverb
let up	No	No	Yes	No
letup	Yes	No	No	No

let up (noun)	letup (noun)
	a pause during which things are calm or activities are diminished

let up (verb)	letup (verb)
become less in amount or intensity
reduce pressure or intensity