verb
guesses; guessed; guessing
to form an opinion or give an answer about something when you do not know much or anything about it [+ obj]
Guess is used in phrases like guess what or guess who as an informal way of indicating that you have surprising news.
[+ obj] :to guess (something) correctly :to make a correct conclusion about (someone or something) by chance
The phrase you guessed it is used as an informal way of saying that what follows is just what you would expect.
[+ obj] chiefly US informal :to suppose or think (something) - usually used following the pronoun I
In responding to a question, the phrase I guess (so) is used as an informal way of agreeing or saying yes when you are not certain or not very excited or interested.
The phrase I guess not is used as an informal way of agreeing with a negative statement or of saying no.
keep (someone) guessing
or chiefly US leave (someone) guessing
to make it impossible for someone to know what will happen next
noun
plural guesses
[count] :an attempt to give an opinion or answer about something when you do not know much about it or are not sure about it
An educated/informed guess is a guess that is probably close to being correct because it is based on some amount of knowledge.
anybody's/anyone's guess
Something that is anybody's/anyone's guess is something that is very uncertain or that no one knows.