Noun. Describes a guy who completely falls for, obsesses over, and acts like an overwrought douche for essentially every girl he meets, who by the way, is undoubtedly, "the one". Typically characteristic of a guy who is single while all of his close friends are in relationships who desperately wants to be hooked up. Usually found cute, at first, by girls he pursues but then is quickly slogged off.
Steve: "Wow! Logan just met that girl last week and he's already talking about moving in together!"
Tom: "I know. He's such a total Mosby."