Often, I hear people say something like this:
You best be going.
You'd best sweep the floor before you leave.
Essentially, you'd best/you best be just means you should. Is it grammatically correct to use these phrases?
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"Had best" is correct, but "had better" is more common. "Has best" is usually associated with dialects, such as in the American South. Source
One can think of them as forms of:
It would be better if you did that.
It would be best if you did that.
"You best" is slang, omitting the 'had', and not correct.
Sentences such as You'd best be going and You'd better be going are fairly common in British English and You best . . . might occur as an elided form of You'd best . . . They are perhaps most frequently found in speech, but I wouldn't be surprised to find even in some academic writing something like I had better set out my position now to avoid misunderstanding later on.
'you best...' (and 'you better...') are both grammatical and productive (you can change the tense and the person).
It's not identical to 'should' but they share a lot in meaning. It often cones in the pattern:
they had better (do something) or (something else unwanted will happen).
It's not very formal so that's why it probably sounds wrong to use in certain contexts.