Green’s Dictionary of Slang

a n.

[abbr. arse n. (1)/ass n.]

1. (W.I., Guyn.) a general term of dislike.

[WI]Allsopp Dict. Carib. Eng. Usage.

2. (US) a euph. for ass n. (4), e.g. haul a, bet your fat a.

[US]W. Winchell On Broadway 9 July [synd. col.] Walter O’Keefe at LaMartinique: ‘She has a broad A — if I make myself clear’.
[US](con. WWII) J.O. Killens And Then We Heard The Thunder (1964) 423: ‘You just take your finger out your ass and make that phone call,’ Samuels said. Solly said ‘Blow it out you “A” bag!’.
[US]‘Richard Hooker’ M*A*S*H (2004) 82: ‘You betcher ever-lovin’ A, buddy-boy,’ Hawkeye said.
[US] (con. WWII) R.H. Sherwood Certified Brave 7: A chorus of jeers quickly erupted interspersed with, ‘Blow it out your “A” and “B” bag,’ and, ‘Aw ya mudder wears army shoes’.

In phrases

get one’s a into g (v.) [abbr. of get one’s ass in gear under gear n., with added element of trying to calculate something]

(N.Z.) to get on with things, to hurry up.

[NZ] in DNZE (1998).
[NZ]T. Reeves ‘A Day at the Beach’ 🌐 A frustrated composer, he married a very arty university woman and looked for the right contacts who would help him get his A into G.
[NZ]McGill Reed Dict. of N.Z. Sl. 9: a into g Arse into gear, usually intended to advise a hurry up. [Ibid.] 86: get your a into g/arse into gear Command to start doing something quickly.