The quality of being witty — of saying clever, funny things. It is the noun for the ability itself, not for one remark. If a friend keeps coming out with sharp funny lines, you can talk about "the wittiness of her replies" or "his easy wittiness". Here is the honest part, and it is the reason to be careful with this word: English already has wit for exactly this idea, and wit is far more common. Native speakers say "her wit", "a man of great wit", "quick wit". Wittiness is real English and it is in dictionaries, but it sounds heavier and more like an essay. It appears most when someone is describing a QUALITY of a piece of work — "the wittiness of the dialogue", "the wittiness of the design" — where wit alone would sound like it belongs to a person. Uncountable: no "a wittiness", no "wittinesses". Formal and rather written. In speech people simply say someone is funny or clever.

機智、風趣（言談用字聰明又好笑的特質）