From Longman Dictionary of Contemporary Englishfatten somebody/something ↔ up phrasal verbFATto make a person or animal fatter My parents are always trying to fatten me up. → fatten→ See Verb table
Examples from the Corpus
fatten up• Will eating carbohydrates fatten you up or trigger diabetes?• We raise calves for sale to feedlots, which fatten them up to market weight. 