A line said by
Hugh Jackman's character
Wolverine in the
X-Men movie.
Mystique, a
shape shifter with the ability to look like anyone she wants had fooled the X-Men previously by appearing as one of the
team. When Wolverine showed up
Cyclops, played by
James Marsden, asked how do they know it's really him. Wolverine's response,
"You're a dick".
Of course I had been using this line for years before the script writer for X-Men stole it from me and made it famous in the movie. It is such a wonderful succinct line that sums up perfectly the feelings I get when someone says something stupid. It's not as harsh as calling someone a fuckhead yet it still gets my point across; you are a dick for doing or saying that and you should think twice about ever repeating that action.
Update:
Thanks to fuzzy and blue for pointing out to me (sourced from an interview in The Onion). It was actually Joss Whedon (of Buffy fame) who was responsible for this line in the movie (and thus of course must have been listening in on one of my conversations!). Apparently the X-Men script was rewritten several times, and this line was one of the few contributions Joss made that survived through to the final draft.