bặt vô âm tín

Etymology
Apparently a Vietnamese construct from Sinitic elements:, ,. Compare the obsolete/archaic compound.

The morpheme is of particular interest; its modern "standard" Sino-Vietnamese reading is  with long vowel. This "four-character compound" is also attested as ; this "alternation" of the first morpheme is also seen in vs., the latter form probably represents contamination by. Furthermore, some dictionaries (including Đại Nam Quấc âm tự vị (1895)) defines bặt as if it is a variant of, which is likely a native word; there was probably some more contamination involved.

Idiom

 * 1) missing; whereabouts unknown; complete radio silent