The release of such an ungodly fart that its potency and duration make those around the farter believe that the farter did not merely fart, but indeed shit his pants. Usually either an insult at the farter's discourtesy in not leaving the room, or a proud boast to indicate the true power of methane.
Fairly ambiguous and is often open to debate on it's correct application.
Historically it has been used to politely ask someone at, say, a formal dinner, if they have farted or indeed, have actually shat their smalls. The use of "have I" helping to ease the embarrasement of the situation.