'stiffy' originates from English, specifically from the adjective 'stiff' with the adjectival/diminutive suffix '-y', where 'stiff' meant 'rigid, firm'.

'stiff' comes from Old English 'stif' meaning 'firm, strong', from Proto-Germanic *stiffaz; the modern English 'stiff' developed through Middle English and later formed 'stiffy' by adding '-y' to create an informal or descriptive form.

Initially, it meant 'rigid, not flexible'; over time the base meaning of 'rigid' stayed but 'stiffy' also took on informal slang senses, including the vulgar noun meaning 'erection'.