As in Pip Pip old chap?
Thought it was an odd term/abbreviation from the beginning.
To some (LFMAO-types) it stands for 'Peeing in Pants'.
Pretty accurate.
Though here's a better one, maybe from the Brit-types in our management globally.
From the OED:
'pip' (tr) British slang
to wound or k–l with a gun;. to attack a person when success seems certain,