Someone who doesn't have the guts to be a full vegetarian but will eat meat once or twice a week. Will still lecture you on the evils of eating meat though.
Laura: I'm a semitarian, I basically just eat vegetables and tofu now, it's so much healthier.
Dave: But don't you have a steak once a week and chicken on Tuesdays?
Laura: Semitarians are allowed that, which is still better than you and your rampant lust for meat.