The word 'what' traces its lineage directly back to the Old English 'hwæt', which served the same core interrogative function. This Old English term originates from the Proto-Germanic 'hwat', itself derived from the Proto-Indo-European root 'kʷod', the neuter form of 'kʷis' (meaning 'who'). This ancient root is the source of interrogative pronouns across many Indo-European languages, such as Latin 'quod' and Sanskrit 'kad'. The phonetic evolution from 'hwæt' to 'what' involved the simplification of the initial 'hw' cluster to 'wh' and later 'w' in some dialects, and the vowel shift. The word's journey is one of remarkable stability, retaining its fundamental role in forming questions and expressing surprise from its earliest reconstructible form to its modern usage.

The word 'wat' is a simplex form. It is a direct descendant of the Old English interrogative pronoun 'hwæt'. Its functional role is as a grammatical word (pronoun/determiner/interjection) used for inquiry and emphasis.