satin, sat in

Satin or satin ( one word )  means the smooth shiny fabric made of silk or of synthetic material--
Her dress was of satin, embroidered with seed pearls and beads.
We are looking for some red satin to make a costume.

Satin is also used to describe something smooth and shiny--
"Out cattin', my satin doll..."
We want some paint with a satin finish.

Sat in or sat in ( two words ) is the past tense ( yesterday, or some time ago ) of "sit", meaning to rest on one's behind "in", meaning in the same space as--
He sat in someone's lunch, and needs to go home to change.
He sat in a puddle; we need to get him some dry clothes.

Now that you know that, you can say--
"She sat in some coffee, and now her satin dress has a big stain."

Note--"sateen" is a cotton or linen fabric with a smooth, shiny finish--
This sateen will make nice curtains.
Note # 2--"Satan" ( one word, always capitalized ) ) is a proper name, meaning the lord of hell or of evil--
 "And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world..."
Not to be confused with "sat an" ( two words ), "sat", the past tense of "sit", meaning to rest upon, "an", meaning one of something that begins with a vowel ( open-mouthed ) sound--
I looked out the window, and there sat an eagle, right on the fence!