double chin vs mentum

double chin and mentum both are nouns.

Word	Noun	Adjective	Verb	Adverb
double chin	Yes	No	No	No
mentum	Yes	No	No	No

As nouns, mentum is a hypernym of double chin; that is, mentum is a word with a broader meaning than double chin:

Other hypernyms of double chin include chin.

double chin (noun)	mentum (noun)
a fold of fatty tissue under the chin	a projection below the mouth of certain mollusks that resembles a chin
	the protruding part of the lower jaw
	a projection like a chin formed by the sepals and base of the column in some orchids