fool around vs joke

fool around is not a noun while joke is a noun.

Word	Noun	Adjective	Verb	Adverb
fool around	No	No	Yes	No
joke	Yes	No	Yes	No

As verbs, joke is a hypernym of fool around; that is, joke is a word with a broader meaning than fool around:

Other hypernyms of fool around include jest, play.

fool around (noun)	joke (noun)
	a ludicrous or grotesque act done for fun and amusement
	activity characterized by good humor
	a triviality not to be taken seriously
	a humorous anecdote or remark intended to provoke laughter

fool around (verb)	joke (verb)
indulge in horseplay	act in a funny or teasing way
commit adultery	tell a joke; speak humorously