There are several antonyms for the phrase "tost off," which means to become heated or cooked quickly. One obvious antonym is "chill," in which an item is made colder instead of warmer. Another antonym could be "undercooked" or "raw," in which an item is not heated enough to be considered cooked. Additionally, "unheated" or "cold" could also be used as antonyms for "tost off." Finally, for fabrics, an antonym for "tost off" would be "crease," in which the fabric does not become crinkled or wrinkled due to heat.