A rose apple of some kind, and that is as far as the naming honestly goes.

The Thai name ชมพู่พลาสติก points two ways at once. Sources that use it give the botanical name Syzygium jambos – the true rose apple, ชมพู่น้ำดอกไม้ – while describing a fruit that matches Syzygium samarangense, the wax apple. The two are different species and the popular names have been traded between them for long enough that the common name settles nothing.

A photograph of a leaf and a ripe fruit would end the question in a minute: S. jambos carries a rounder, hollow fruit with a rose scent and a loose seed that rattles, while S. samarangense is the flatter bell shape with almost no seed at all. Until then this entry stays a guess and says so.