lowball vs underestimate

lowball is not a noun while underestimate is a noun.

Word	Noun	Adjective	Verb	Adverb
lowball	No	No	Yes	No
underestimate	Yes	No	Yes	No

As verbs, lowball and underestimate are synonyms defined as:

lowball (noun)	underestimate (noun)
	an estimation that is too low; an estimate that is less than the true or actual value

lowball (verb)	underestimate (verb)
make a deliberately low estimate	make a deliberately low estimate
	make too low an estimate of
	assign too low a value to