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3.
A statement made when somebody mentions the improbable.
Lame Person Who neithers Bathes nor Thinks- Duuuhhh...so when can I get your phone number?

Me- Oh, let's see...how about when pigs fly?
by RatchetBoo June 06, 2003
 
1.
never; it won't happen
F student: Think I could get into Harvard?
A student: When pigs fly!
by William MacCabe November 24, 2005
 
2.
Sacrastic sentence suffix to indicate non-adherance to the subject in hand.
Sure I'll pay you... when pigs fly!
by Gumba Gumba February 25, 2004