The phrase "tuck in" usually means to tuck in one's shirt or to eat heartily. Some common antonyms for the first usage of "tuck in" include "untuck," "drape," or "hang loose." For the second usage, antonyms could include "refrain," "fast," or "starve." However, it's worth noting that the phrase "tuck in" can also be used to mean "put to bed," in which case some antonyms could include "awaken," "disturb," or "rouse." As with any word or phrase, context is important when looking for antonyms, as there may not always be an exact opposite.