A deformed or swollen hand, or a person with such a hand; historically, an insult referring to someone with a clenched fist or awkward hand.
From 'club' (the shape) plus 'fist' (closed hand). Used since at least the 16th century in medical contexts to describe hand deformities and sometimes as a derogatory term.
Like 'clubfoot,' this shows how old English wasn't squeamish about naming disabilities directly after the thing they resembled—people with hand deformities were called 'clubfists,' which sounds harsh now but was just literal description then.
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