underestimate vs guess

underestimate and guess both are nouns.

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

As verbs, guess is a hypernym of underestimate; that is, guess is a word with a broader meaning than underestimate:

Other hypernyms of underestimate include approximate, estimate, gauge, judge.

underestimate (noun)	guess (noun)
an estimation that is too low; an estimate that is less than the true or actual value	an estimate based on little or no information
	a message expressing an opinion based on incomplete evidence

underestimate (verb)	guess (verb)
make a deliberately low estimate	expect, believe, or suppose
make too low an estimate of	judge tentatively or form an estimate of (quantities or time)
assign too low a value to	guess correctly; solve by guessing
	put forward, of a guess, in spite of possible refutation