Definition of fattens:
(v) :
(transitive) To cause (a person or animal) to be fat or fatter.
(v) :
(intransitive, of a person or animal) To become fat or fatter.
(v) :
(transitive) To make thick or thicker (often something containing paper, especially money).
(v) :
(intransitive) To become thick or thicker.
(v) :
(transitive) To make (soil) fertile and fruitful.
(v) :
(intransitive) To become fertile and fruitful.