ETYM Old Eng. tarte, French tarte; perhaps originally the same word as tourte, Late Lat. torta, from Latin tortus, p. p. of torquere to twist, bend, wind, because tarts were originally made of a twisted shape. Related to Torture.
Small open pie.
A dish baked in a pastry shell; pie: as; a small pie or pastry shell without a top containing jelly, custard, or fruit; a small pie made of pastry folded over a filling.

ETYM as. teart. Related to Tear.
1. Sharp to the taste; acid; sour.
2. (Figurative) Sharp; keen; severe.