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1. rude slang Go away. ("Eff" in these headwords is the phonetic spelling of the letter F, representing the word "fuck.") Eff off, will you? I don't want to hear any more of your crap. A: "Tom wants to ask you some more questions about the project." B: "Tell him to eff off, would you? I'm busy."
2. rude slang To engage in aimless recreation or frivolous time-wasting. You'd be done with your homework by now if you didn't spend so much time effing off on that phone of yours.