A person with whom the speaker has a highly intimate relationship both physically and emotionally, often sharing ideas and actions that are only between the two of them. Not usually applied to significant others, and implies the possibility of multiple partners of the same type. Similar to lover, but with a connotation of strong friendship and often mischievous secrecy as well.
Chris is my doxy. He's one of my closest friends and I enjoy hooking up with him, but we're definitely not in a relationship and it definitely isn't just hooking up.