A hard-on is a very informal, rude word for when a man's penis is stiff because he is sexually excited — an erection. Doctors and school books say erection.

A hard-on ishard-on 是一個非常口語、粗俗的說法，a very informal, rude word for用來說 when a man's penis is stiff because he is sexually excited男生因為性興奮而陰莖變硬的狀態—— an erection也就是「erection」（勃起）。Doctors and school books say醫生和教科書會說 erectionerection（勃起）。

This word lives in jokes, films, locker rooms, and private talk among friends, not in class, emails, or a doctor's room. Students often meet it in songs and subtitles long before they need it, and they do not need it to be polite. It names a male body fact; it is not a careful or medical term.

This word lives in jokes, films, locker rooms, and private talk among friends這個詞會出現在笑話、電影、更衣室、朋友間的私下對話裡，not in class, emails, or a doctor's room不會出現在課堂、電子郵件或診間裡。Students often meet it in songs and subtitles學生常常在歌曲和字幕裡先碰到這個詞 long before they need it早在他們真正需要用到它之前，and they do not need it to be polite而且他們不需要用這個詞來表現禮貌。It names a male body fact它指的是一個男性身體上的事實；it is not a careful or medical term不是謹慎或醫學上的用詞。

Though actually, a different phrase looks almost the same and tricks people all the time: to be hard on someone means to treat them strictly, with no sexual meaning at all. The little line in the middle, and the a (a hard-on), mark the slang noun.

Though actually不過其實，a different phrase looks almost the same and tricks people all the time有一個長得幾乎一樣的片語常常騙倒大家：to be hard on someone'to be hard on someone' means to treat them strictly意思是對某人很嚴格，with no sexual meaning at all完全沒有性方面的意思。The little line in the middle中間的那個連字號，and the a (a hard-on)還有前面的 a（a hard-on），mark the slang noun就是在標記這是俗語名詞的用法。