When a random person sits with your group while your eating anywhere, usually taking up the LAST seat. Fish will not say a word, no matter what. Fish sits there and looks stupid while everyone bitches at him to move, but wont because he knows he took the last seat ANYWHERE, excluding one of your friends.
Timmy got held up in class and went to lunch late, only to find a fish in his seat.
Timmy- gtfo my seat...
everyone- Fish, out!!