what

An expression of surprise or disbelief.

What do you want? An abrupt, usually unfriendly enquiry as to what a person desires.

(Britain, colloquial, dated) Clipping of what do you say? Used as a type of tag question to emphasise a statement and invite agreement, often rhetorically.

What did you say? I beg your pardon?

Indicating a guess or approximation, or a pause to try to recall information.