This word usually lets the other person know where you stand with their suggestion / comment / plan.
Usually used after the companion says the offending sentence but in enxtreme cases you may interuppt and say loudly and proudly "Giller"
You may also optionally add either "Thriller" or "Killer" to further add to the situation.
Person A: I think you should lend me a ciggerette.
Person B: No i always lend you them, you never have any
Person A: Come on i always give you them back.
Person B: Oh for god sake your a giller
Person A: I'm going to mess around with your sis....
Person B: YOU GILLER KILLER / YOU GILLER THRILLER
Person A: ...tem