by guess and by God
By any means, typically those that lack careful planning or rely on guesswork and luck. We don't have much to work with, but by guess or by God we're going to figure out a way to save our business! He just throws his essays together by guess or by God, but he still manages to get good grades on them.
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 alternative by guess and by Godfrey is also sometimes found.