onanist vs jerk-off

onanist and jerk-off both are nouns.

Word	Noun	Adjective	Verb	Adverb
onanist	Yes	No	No	No
jerk-off	Yes	No	No	No

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

Other hyponyms of onanist include frotteur, tosser, wanker.

onanist (noun)	jerk-off (noun)
a person who practices masturbation	terms of abuse for a masturbator