A stiffy is an erection — when a man's penis becomes hard. It is a noun, countable: "a stiffy", "to get a stiffy", "to have a stiffy". This is very informal slang, mostly British and Australian, and it is usually said as a joke or between close friends. It is not rude in the strongest way — it is a playful, slightly childish word rather than a shocking one — but it is still a word about sex, so it does not belong in class, at work, in writing, or with people you do not know well. You will meet it in comedies and casual speech. The neutral word for the same thing is "an erection", and that is the one to use anywhere serious, including at a doctor's. There is one other, harmless use worth knowing: in British English a stiffy can also mean a stiff card invitation to a formal event, and in older South African computer talk it meant a floppy disk. Those are rare, and the sexual meaning is what people will hear first.

勃起（俚語、玩笑口吻）。正經場合要說 erection，中文正式說法是「勃起」