Best definition
wechby
An acronym for “Woman Everyone Can Have But You”.
Suggesting someone who is an object of fascination and/or obsession, but unobtainable.
Possibly

first coined to refer to an archetype in the Unknown Armies roleplaying game.

Example:

Character 1: OMG! Did you see that chick on the bus? She’s got everything I need in a woman.

Character 2: Yah, I’ve seen her around alot before. But seems like she’s always in the company of some other guy. I pined after her awhile myself, but eventually just wrote her off as a total WECHBY.