SAY WITHOUT KNOWING [I, T] › 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] › 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 › 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 › used when you agree/disagree but are not completely certain about something
Guess what? › something you say when you have some surprising news for someone: Guess what? I'm pregnant.