What is the full form of 'WHAT'?

The Word 'what' is the full form of that Word.

It seems to have meant 'what', as in 'what is that?' for about as far back as we can trace it.

The sense of 'what' as in 'what (did you say?)' goes back in English to around 1300; as an end to a question, 'Shall we go, or what?' meaning, 'Shall we go, or do something else?' goes back to the late 1700s.