suck vs suck in

suck is a noun but suck in is not a noun.

Word	Noun	Adjective	Verb	Adverb
suck	Yes	No	Yes	No
suck in	No	No	Yes	No

As verbs, suck and suck in are synonyms defined as:

suck and suck in: attract by using an inexorable force, inducement, etc.

suck (noun)	suck in (noun)
the act of sucking

suck (verb)	suck in (verb)
give suck to	take up as if with a sponge
draw into the mouth by creating a practical vacuum in the mouth	draw in as if by suction
take in, also metaphorically	attract by using an inexorable force, inducement, etc.
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.