guess
anyone's (or anybody's) guess something very difficult or impossible to determine. informal
2002Jeffrey EugenidesMiddlesex Where Lefty obtained the hash is anybody's guess.
by guess and by God without specific guidance or direction.
☞ This expression was originally used in a nautical context, where it meant to steer blind, without the guidance of landmarks. The euphemistic alternative by guess and by Godfrey is also sometimes found.
keep someone guessing leave someone uncertain or in doubt as to your intentions or plans. informal
there are no prizes for guessing: seeprize.
your guess is as good as mine I know as little about the matter as you (used in answer to a question).