put (weight) on
Definition: to gain weight; to become heavier.
(This phrasal verb has more than one meaning)
E.g.1: I must stop eating chocolate biscuits – I’ve put 5kg on since the beginning of the year.
E.g.2: Why is it that putting weight on is so easy, but losing weight is so difficult?
This phrasal verb can be separated (E.g. 1 & 2).