From the saying "That's a Mact". Basically, it's a fact which is of doubtful authenticity and may well have been completely made up by anyone named Matt.
Matt: "I drive an Audi TT, am half-Columbian and have awesome dimples."
Everyone else: "And that's a Mact!"
Everyone else: "And that's a Mact!"
by Lord Tennyson January 01, 2012