This expression is from the movie There's Something About Mary:
She put a lot of weight, about a deuce and a half.
Is this used to refer to overweight people? The only reference I could find is from some American Army truck.
Learn English – “Deuce and a half” for body weight
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Best Answer
The expression is from military slang for a large truck — that weighs two (a deuce) and a half tons.
I hadn't come across this expression before, but I have to say it doesn't seem very apposite to me. My car weighs nearly 2 tons, so a 2.5-ton truck doesn't sound particularly big to me. Wikipedia agrees, saying there are 8 truck categories — the lightest of which is for anything under 3 tons.
Anyway, it's not exactly "common", but here are a few dozen instances of "she's a deuce and a half" showing that it's far from unknown (even if the scale factor is a bit off).
Per comments below — unquestionably "a deuce and a half" is military slang, but it refers to the load-carrying capacity of the truck, not total weight. And Urban Dictionary's 2.5 x 100lbs = 250lbs is, well, an urban legend sort of rationalisation.