What it means
A headcase is someone whose behaviour is a bit unhinged, wild, or unpredictable, the mate who does daft things for a laugh or the person you genuinely worry about. It swings from affectionate ribbing to real concern depending on the tone.
Usage examples
"Don't dare him to do it, he's a headcase and he actually will."
"He jumped in the canal in January, absolute headcase."
Where it comes from
Pictures the head as a "case" to be examined, the way you would talk about a medical or psychiatric case. Calling someone a headcase says their head is the bit that wants looking at, half joke, half jab.
Other ways to say it
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