to gain weight, to get fat
Tom no quiere comer tarta porque no quiere engordar más.
Tom doesn't want to eat cake because he doesn't want to gain any more weight.
Don’t let yourself gain weight.
Los podríamos engordar con la porquería de tus sucios pies.
We could fatten them up with the filth from your dirty feet.
— El Submarino (1981)
Engordar demasiado no es bueno para la salud.
Gaining too much weight is not good for your health.