suck vs go down on

suck is a noun but go down on is not a noun.

Word	Noun	Adjective	Verb	Adverb
suck	Yes	No	Yes	No
go down on	No	No	Yes	No

As verbs, suck and go down on are synonyms defined as:

suck and go down on: provide sexual gratification through oral stimulation

Other synonyms of suck include blow, fellate.

suck (noun)	go down on (noun)
the act of sucking

suck (verb)	go down on (verb)
give suck to	provide sexual gratification through oral stimulation
draw into the mouth by creating a practical vacuum in the mouth
take in, also metaphorically
draw something in by or as if by a vacuum
provide sexual gratification through oral stimulation
be inadequate or objectionable
attract by using an inexorable force, inducement, etc.