My favourite: "Onageristic Estimate"
Onager is an animal related to the donkey/horse. Dictionary describes it as a wild-ass or half-ass, therefore, onageristic estimate = wild-ass guess or half-ass guess.
I know for a fact that ACAS Ops in MOD used it twice in letters he signed (tee hee). Had to do something to keep you sane while in the funny farm.
