member vs hard-on

member and hard-on both are nouns.

Word	Noun	Adjective	Verb	Adverb
member	Yes	No	No	No
hard-on	Yes	No	No	No

As nouns, hard-on is a hyponym of member; that is, hard-on is a word with a more specific, narrower meaning than member:

Other hyponyms of member include cock, dick, pecker, prick, putz, shaft, tool, micropenis, microphallus, erection.

member (noun)	hard-on (noun)
an external body part that projects from the body	an erect penis
the male sex organ
an organization that is a member of another organization (especially a state that belongs to a group of nations)
one of the persons who compose a social group (especially individuals who have joined and participate in a group organization)
anything that belongs to a set or class