Green’s Dictionary of Slang

(US black) the washing by a woman of only the genital area, the breasts and armpits, all of which are most likely to smell.

1971		E.E. Landy Underground Dict. (1972).
1980		E. Folb Runnin’ Down Some Lines 144: p.t.a. used when a female only washes ‘pussy, titties, and armpits’.
2002		J. Masterton In This World 🌐 Ch. i: Swiftly rubbing the white suds under Clara’s arms, Ursla spoke as if reading from the recipe of a favorite family dish. ‘We call this a PTA bath: pits, tits and ass, that’s all you got to do.’.
2002		Rupaul 10 Mar. [blog] My call time on SON/BEACH was 6:30, so in order to take a PTA bath (pussy, tittie and ass), I had to get up at 5am!