fattenEnglishOxford American Thesaurusfatten verb 1 a farm where livestock are fattened: make fat/fatter, feed up, feed, build up; overfeed, bloat. 2 you need to fatten up: put on weight, gain weight, get heavier, grow fat/fatter, get fat, flesh out, fill out; thicken, widen, broaden, expand, spread out. ▷antonyms lose weight.