gag vs thigh-slapper

gag and thigh-slapper both are nouns.

Word	Noun	Adjective	Verb	Adverb
gag	Yes	No	Yes	No
thigh-slapper	Yes	No	No	No

As nouns, thigh-slapper is a hyponym of gag; that is, thigh-slapper is a word with a more specific, narrower meaning than gag:

gag (noun)	thigh-slapper (noun)
restraint put into a person's mouth to prevent speaking or shouting	a joke that seems extremely funny
a humorous anecdote or remark intended to provoke laughter

gag (verb)	thigh-slapper (verb)
struggle for breath; have insufficient oxygen intake
make an unsuccessful effort to vomit; strain to vomit
cause to retch or choke
make jokes or quips
be too tight; rub or press
tie a gag around someone's mouth in order to silence them
prevent from speaking out