Fat leak is a term used to describe a bulge of bare flesh that the owners clothes do not (but should) cover. Fat leaks often occur at the join between the upper and lower garments. Remember, a fat leak is only when the fat appears to seep out as uncovered flesh and can't be used to describe covered bulges. This is a derogatory term and it will not work as a term of endearment.
"Mary you have a fat leak" or "look at Mary's fat leak"