ấy

Etymology
Diachronically, this is probably a Northern Vietnamese innovation occured some time between the stages of Proto-Vietnamese and attested Northern Old Vietnamese. It is amply attested in the oldest Nôm texts (which are all in Northern Old Vietnamese), yet has no cognate in the Muong lects, and usage of this morpheme by Central and Southern speakers clearly indicates borrowing. It also does not neatly fit into Proto-Vietnamese demonstrative paradigm with its zero-initial.

The situation can probably be hypothesized as:

Determiner

 * 1)  that; those

Particle

 * 1)  sentence-ending intensifying particle

Pronoun

 * 1)  you, my buddy/gal