Wiktionary
vb. (context transitive English) To cause to weight gain by means of feeding.
WordNet
Usage examples of "fatten up".
Reverend Baldwin gave me a clean bill of health three weeks ago, said my leg was mending just fine, then told me to fatten up before I came here.
I hoped that you could stay with us for a few days and fatten up your Volodia.
So if the worshipers were prepared to tithe sufficient donations for the priesthood to fatten up sufficient sacrificial animals to scent that much smoke, then they must be really devout.
If the crops this year were good, if the alfalfa and oat hay grew thick and rich, there would be enough money and feed to buy and fatten up beef calves in the autumn.
And, while it was off the beaten track, it would be good to leave some cattle there to fatten up.
If the forage happened to be bad alongside the track that led to his domain, he preferred losing a few beasts to turning aside to let them fatten up.