Dead bodies, or people who are very formal and unfriendly, or in slang, people who don't tip or pay what they owe.
From Old English 'stif' meaning rigid or inflexible. The term gained the slang meaning of 'corpse' from the rigidity of death, and later expanded to describe anyone who moves or acts rigidly.
The word 'stiff' connects death, formality, and dishonesty through the image of rigid inflexibility—your body becomes stiff in death, you stay stiff (formal) when uncomfortable, and you're 'stiff' (stubborn/unreliable) when refusing to cooperate.
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