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1.
Verb. "to mosby someone". Telling someone (usually on your first date) that you love them in order to freak them out and deter them from having a further relationship with you.

Originates from the show How I Met Your Mother.
Robin: "I found out Barney loves me, what am I gonna do? It's gonna break his heart when I tell him I don't love him back."

Marshall: "Well, there's an option. High risk but high reward. You could mosby him."

Lily: "Oh no, she couldn't mosby him."

Marshal: "She could MOSBY THE CRAP OUT OF HIM."
by K4m1k4z3 May 26, 2009
 
2.
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."
by Becky the Chick April 22, 2011
 
3.
a mosby is, an incredibly good looking person. thats most of the time not the tallest person, generally likes to be center of the attention, and or can have shorter hair.
That girl is a mosby.
by -Dr.Person April 17, 2010