A warning that first appeared in Ultima IV: Quest of the Avatar. Earning all eight pieces of the ankh is a major part of the aforementioned quest. You have to act in harmony with all eight virtues in order to earn each piece. If you act unvirtuously, by cheating a blind herb-seller, attacking a peaceful citizen, etc., you will lose that virtue. If you've earned that virtue's piece of the ankh, you will also lose that eighth.
On the Ultima-related newsgroups, "Thou hast lost an eighth!" is used as a rebuke when someone asks a stupid question.
"Thou hast lost an eighth" also appears in Doom II -- when you want to quit the game, the confirmation dialog occasionally warns you that "Thou hast lost an eighth" for wanting to quit!