kowtow vs suck up

kowtow is a noun but suck up is not a noun.

Word	Noun	Adjective	Verb	Adverb
kowtow	Yes	No	Yes	No
suck up	No	No	Yes	No

As verbs, kowtow and suck up are synonyms defined as:

Other synonyms of kowtow include bootlick, fawn, kotow, toady, truckle.

kowtow (noun)	suck up (noun)
a former Chinese custom of touching the ground with the forehead as a sign of respect or submission

kowtow (verb)	suck up (verb)
try to gain favor by cringing or flattering	try to gain favor by cringing or flattering
bend the knees and bow in a servile manner	take in, also metaphorically
	ingratiate oneself to; often with insincere behavior