Fat  —n. 1 natural oily or greasy substance found esp. In animal bodies. 2 part of meat etc. Containing this. —adj. (fatter, fattest) 1 corpulent; plump. 2 containing much fat. 3 fertile. 4 a thick (fat book). B substantial (fat cheque). 5 colloq. Iron. Very little; not much (a fat chance; a fat lot). —v. (-tt-) make or become fat.  the fat is in the fire trouble is imminent. Kill the fatted calf celebrate, esp. At a prodigal's return (luke 15). Live off (or on) the fat of the land live luxuriously.  fatless adj. Fatness n. Fattish adj. [old english]