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1.
To ask someone to explain something.

The first known occurence of this bastardization of the word "explain" is found in the "Chosen" episode of the television series "Buffy the Vampire Slayer".

Currently in heavy usage on message boards everywhere.
ANGEL: I brought something else as well.

(He removes a silver amulet with a multi-faceted crystal at its center from his pocket.)

BUFFY: I can already tell you, I have nothing that goes with that.

ANGEL: It’s not for you.

BUFFY: Splainy?
by pareidolic May 06, 2004
 
2.
explain, please? expression of bafflement, request for information. from 'explain'.
A: Of course, that might indicate a negative correlation between X and....
B...splainy?
by melaka67 October 15, 2007