Another, more slangy version of saying "Fuck that
Shit!".
Usually only used when speaking to someone you know, and
sometimes used with "Dude" proceeding it.
Almost exclusively used when honestly hating an idea or concept, but not used in a sexual sense at all.
Johnny: "Hey, lets go to the movies AFTER doing our
homework.
Tommy: "Dude, F that Ish! I say we go to the movies first, and F the
homework!"