waist, waste

Waist or waist means the part of the body or clothing where the pants or skirt meet the shirt--the middle, where the navel is--
These pants are too big in the waist.
Women used to wear corsets to make their waists smaller.

Waste or waste ( rhymes with waist ) means to throw something useful away, to spoil it or allow it to be spoiled--
Finish your dinner--we don't want to waste food.
They forgot to put it in the refrigerator, and it all went to waste.

Now that you know that, you can say--
"She often let her dinner go to waste, in hope of keeping her slender waist."

Note--a "wastrel" is a good-for-nothing, or someone who wastes money--
He ran away from home and became a drunkard and a wastrel.