this is a shorter term for "Flap Of Skin". a flap of skin is someone in your group of friends whom you may consider the "loser" or the person who you only hang around with for a certain reason, that reason not perticulary meaning you enjoy their company.
it could also mean someone with NO private parts.
" Kris has no vagina"
"She is but a Flap Of Skin!"
"I couldn't concentrate when the FOS was talking to me, she annoys me to much."