Englishified Greek for "Come and get it." Pronounced "Moh-lone Lah-Veh". Common context is in relation to firearms and firearm laws, as a declaration to be made to anyone that wishes to confiscate your gun(s).
Originates from King Leonidas of Sparta, in a message to King Xerxes of Persia, when an overwhelming Persian army demanded that 300 Spartan warriors drop their weapons. Leonidas replied "Molon Labe" (Come and get them) and 300 of his Spartans, and Leonidas himself, were killed, but in so doing killed took huge tolls on the enemy and stalled the Persian army for long enough to let the rest of Greece prepare themselves for the war.
If someone comes to take your guns, tell them "Molon Labe".