fall out vs hap

fall out is not a noun while hap is a noun.

Word	Noun	Adjective	Verb	Adverb
fall out	No	No	Yes	No
hap	Yes	No	Yes	No

As verbs, fall out and hap are synonyms defined as:

Other synonyms of fall out include come about, go on, happen, occur, pass, pass off, take place.

fall out (noun)	hap (noun)
	an accidental happening

fall out (verb)	hap (verb)
come to pass	come to pass
have a breach in relations
drop out
leave (a barracks) in order to take a place in a military formation, or leave a military formation
come as a logical consequence; follow logically