Ace
I think most of the 'backronysms' have been covered, apart from this one for A Crappy Effort.

Also, someone mentioned chinny reckon, which evolved into chinder (emphasis on 2nd syllable) at my school.
( tangledupinblue hairy badge with moving eyes, Fri 6 Jan 2012, 16:15, 3 replies, latest was 14 years ago)

Jimmy
HIIIIIIIIIIIIILLLLL!
( Devil_In_Tights the *real* DiT!, Fri 6 Jan 2012, 16:18, Reply)

We also used to unintelligibly mutter "oh, yes" whilst doing the hand chin action.

(Set your faces to Stunned Bigly, Fri 6 Jan 2012, 16:18, Reply)

Bonus points
For how long you could be nodding and chin-stroking before the other person noticed.
( tangledupinblue hairy badge with moving eyes, Fri 6 Jan 2012, 16:21, Reply)

Hahahaha!
If it was an all out assault people you weren't even looking at were doing it behind your back.
(Set your faces to Stunned Bigly, Fri 6 Jan 2012, 16:23, Reply)

Oh man this takes me back.

( Monty Boyce, My cheese game is strong, Fri 6 Jan 2012, 16:27, Reply)

The shame felt on realising
Might have prompted someone to lick their finger then mime touching your face with an accompanying hiss sound to demonstrate how hot your red face was.
( tangledupinblue hairy badge with moving eyes, Fri 6 Jan 2012, 16:29, Reply)

Hahahaha fuck me this is memory magic for me

( Monty Boyce, My cheese game is strong, Fri 6 Jan 2012, 16:42, Reply)

'begin again'
from Michael Finnigen.

Which is really fucking lame, with hindsight.
( scarpe We Stole Bikes, Fri 6 Jan 2012, 16:21, Reply)