onanist vs frotteur

onanist and frotteur both are nouns.

Word	Noun	Adjective	Verb	Adverb
onanist	Yes	No	No	No
frotteur	Yes	No	No	No

As nouns, frotteur is a hyponym of onanist; that is, frotteur is a word with a more specific, narrower meaning than onanist:

onanist: a person who practices masturbation
frotteur: someone who masturbates by rubbing against another person (as in a crowd)

Other hyponyms of onanist include jerk-off, tosser, wanker.

onanist (noun)	frotteur (noun)
a person who practices masturbation	someone who masturbates by rubbing against another person (as in a crowd)