What it means
A lightweight is someone who can't handle much, classically booze, the friend who's giggling and sleepy after a single pint while everyone else is just warming up. By extension it's anyone who taps out early or can't keep up, the one who folds at the first hurdle. Said with a grin, half mockery and half envy of their cheap night out.
Usage examples
"Two drinks and he's done, what a lightweight."
"I'm such a lightweight these days, one glass of wine and I'm ready for bed."
"Don't invite Jess to the afters, she's a total lightweight and starts yawning halfway through round two."
"Used to do all-nighters, now I'm a proper lightweight after one cocktail and a chip."
Where it comes from
It started life in boxing, where a lightweight is a fighter in a lower weight class. From there it slid into everyday English and got repurposed for someone who can't handle much, especially alcohol. Same word, new arena: less about the scales, more about folding after one pint.
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