The slang meaning of "Seven/七" in Cantonese
It is a foul character, usually pronounced as "cat6".

The original character is "", which means "scrotum".  On the contrary, in modern slang uses, it refers to the penis in a flaccid state, and commonly written as "" or "柒".  The implied meaning is thus "useless", "stupid", etc.

Many people tweak the pronounciation from "cat6" to "cat1" (hence, "七"/"柒") to avoid speaking the foul character directly.  Other alternatives like "sat6" (實) are also used.