A day of the week which appears only when something happens that usually would never happen such as a blackhole, division by zero, learning to carry pineapples with your mind, etc. Mursdays have never appeared in the history of mankind.
When that loser Ebert finally got laid, everyone thought that the next day had to be Mursday, however, it was only hell that froze over.
used in im conversations to say "insert sarcasm here." Used by some to say things that would not be understood if not literally said or would be taken out of context.
Person 1: I just did Tanya
Person 2: The fat one? good job! (I-sarcasm-H).