The phrase is "my foot my tutor?", and comes from Shakespeare's Tempest. I think in a figurative sense, the foot is trying to give advice to the head. You could understand it as "Has my foot become my tutor now?" In the play, Miranda starts advising her father Prospero at the exact moment Prospero is trying to exercise his power over Ferdinand. Bad timing: Prospero is shocked that his daughter has chosen to be an upstart and (seem to) give advice which suits only herself. It's not really related to the expression "My foot!", which came several centuries later.