From Wiktionary, the free dictionary
From thả (“to drop; to release”) + tim (“heart”); based on the slang term thả thính.
thả tim
- (neologism, slang) to send out or show one's affection, such as by giving a heart react on Facebook, making a finger heart gesture, etc.
- Auto thả tim Facebook ― plugin which automatically gives a heart reaction to others' posts on Facebook