Meaning of “guess” - Learner’s Dictionary

guess

verb uk us /ɡes/
SAY WITHOUT KNOWING [ I, T ]

A2 to give an answer or opinion about something without having all the facts:

Can you guess how old he is?
I would say he probably works in the media, but I'm just guessing.
SAY CORRECTLY WITHOUT KNOWING [ I, T ]

A2 to give a correct answer without having all the facts:

"You've got a promotion!" "Yes, how did you guess?"
[ + (that) ] I'd never have guessed that you two were related.
I guess

B1 used when you think that something is probably true or likely:

I've known her for about 12 years, I guess.
I guess so/not

B2 used when you agree/disagree but are not completely certain about something

Guess what?

A2 something you say when you have some surprising news for someone:

Guess what? I'm pregnant.

(Definition of “guess verb” from the Cambridge Learner’s Dictionary © Cambridge University Press)