BT Ber 56a(40) :א׳ל ו׳ו גהיט עיין רבא וחזא דגהיט ו׳ו דתפיליה‏ : He said to him: The waw is erased. PN looked carefully and saw that the waw of his phylacteries was erased
See the entire entry for ghṭ vb. to erase