- "come off it" は「ばかなこと言うなよ／ばかな考えはよせよ／そんなこと言うなよ／やめろよ」と、人の意見をさえぎる時や、人の言うことを信じてない時に使う表現。（to stop talking or acting in a foolish way ― usually used as an interjection／used to tell someone that you do not believe what they are saying／stop saying or doing that／used to disagree with somebody rudely／used to tell someone that you do not believe what they are saying is true, or that you strongly disagree with them／ to stop being wrong, foolish, or pretentious; be truthful or honest）
- I could be a pro golfer if I really tried.---Oh, come off it! You're not even close to being that good!
- Oh come off it! You can't seriously be saying you knew nothing about this.
- Come off it, Pete, there's no reason to be scared of asking her for a date.
- Come off it! We don't have a chance.
- Ask Simon to cook the meal? Come off it, he can hardly boil an egg!
- Come off it―we know you're as poor as the rest of us.
- ドラマ「フルハウス」のジョーイの決め台詞の"cut it out"と同じような感じなのかな？と思って調べてみた。"cut it out"を英英辞書で確認すると、「used to tell someone to stop doing something because it is annoying you／used to tell somebody to stop doing or saying something annoying」とある。"come off it" には、相手の行動に同意しないぞ、という気持ち（strongly disagreeとかdo not believeとか）が背景があるのに対して、"cut it out" は、相手の言動にイラッときてる、煩わしい（annoying）からやめさせたい、という気持ちが背景にあるのが違いのよう。cutという単語の持つ、バサッとした印象が感じられる。
- Hey, you guys, cut it out - Mom's trying to get some sleep.
- That's enough, kids. I mean it! Cut it out!
- His friends kept fooling around, and he kept hoping they'd cut it out.
- I'm sick of you two arguing―just cut it out!
- この "come off it" と "cut it out" のニュアンスの違いを調べていたら、こちらのサイトがわかりやすかった。
（Philosopher's Stoneがイギリス版、Sorcerer's Stoneがアメリカ版。）