Making or becoming fat or thicker; gaining weight or adding weight-increasing substances to something.
From Old English 'fæt' (fat/oil), with the suffix '-en' (to make) and '-ing' (continuous action); the root is Proto-Germanic, possibly related to eating rich food.
The 'fattening' process in agriculture—giving cattle rich grain just before slaughter—actually became controversial because it creates different meat quality but also ethical concerns, which is why grass-fed beef markets use 'never fattened' as a selling point.
Complete word intelligence in one call. Free tier — 50 lookups/day.