The word 'too' is a specialized spelling variant of the preposition 'to', which has its roots in Old English 'tō', meaning 'in the direction of, for the purpose of, furthermore'. This Old English term is traced back to Proto-Germanic 'tō', itself from the Proto-Indo-European demonstrative base 'do-', meaning 'to, toward'. By the 16th century, the stressed form of 'to' used to mean 'in addition, as well, moreover' began to be spelled 'too' to distinguish it from the preposition. The sense of 'excessively' developed from the idea of going 'to' an extreme degree, completing its logical journey from a directional marker to an intensifier and additive adverb.

The word 'too' is a simplex form. It is a variant of the preposition/adverb 'to', originating from Old English 'tō'. Its functional role is as an adverb.