Where does “người thứ ba” come from?

người thứ ba (Vietnamese) comes from Vietnamese thứ ba, from Vietnamese bà, from Chinese 爸, from Chinese 父, from Komodo paʔ, from Proto-Malayo-Polynesian əpat, from Proto-Austronesian Səpat — four.

người thứ ba (Vietnamese): the third person; the other man or woman;...

Definitions

the third person; the other man or woman;...

Ancestry of “người thứ ba”, step by step

người thứ ba traces back through two separate lines of ancestry.

via Vietnamese thứ ba

Step	Language	Word	Meaning
1	Vietnamese	thứ ba	third; Tuesday
2	Vietnamese	bà	a grandmother; an old woman; an old lady; you, my...
3	Chinese	爸	—
4	Chinese	父	—
5	Komodo	paʔ	four
6	Proto-Malayo-Polynesian	əpat	four
7	Proto-Austronesian	Səpat	four

via Vietnamese Người

Step	Language	Word	Meaning
1	Vietnamese	Người	You/Thou/Thee, Jesus Christ; You/Thou/Thee, my...
2	Proto-Vietic	ŋaːj	—
3	Proto-Mon-Khmer	(m)ŋaaj	person, human being; human being; person

Every word from Proto-Austronesian Səpat →Every word from Proto-Malayo-Polynesian əpat →Every word from Komodo paʔ →