sack, sac

Sack or sack means a bag--for most purposes a cloth drawstring bag, but in some regions any kind of a bag--
Santa will be packing his sack of toys onto his sleigh any day now.
The empty flour sack made a useful kitchen towel.

Sack also means to pillage, as a defeated city in wartime--
The general would not permit his troops to sack the city.

Sac or sac ( rhymes with sac ) is a biology term, meaning a "bag", as of a bag of fluid in an animal or plant--
Under the flower is a sac full of nectar.
This fish has a special sac that fills with air.

Now that you know that, you can say--
"The sac under its wing resembles a sack full of oranges."

Note--"sacque" ( rhymes with sack ) means a sort of dress shaped like a bag--
Her party dress was a sacque of pale blue silk.
Note # 2--"sack" is also an old-fashioned word for sherry or fortified wine--
They enjoyed a glass of sack before dinner.
Note # 3--"Sac" ( always capitalized ) is the name of an American Indian tribe--
The Sac or Sauk Indians still live in Iowa.