Flirtatiousness is the quality of behaving in a way that deliberately tries to attract sexual or romantic attention — the habit or the air of it, not one single joke. It is a heavy, rather formal noun. Magazines and critics use it; friends almost never say it about someone standing in front of them, because it can sound like a label stuck on a person, especially a woman. Often the simpler words are kinder and truer: she was flirting, or there was some flirting between them. The noun makes a manner sound like a fixed trait, and that is frequently more than we actually know. Same warning as the adjective: do not confuse ordinary warmth with this quality.