The phrase "fake mustache" refers to a false or pretend mustache that someone wears on their upper lip. It is not a real mustache and is often used as a disguise or to play a prank on someone. Full definition
Members of Trotman don fake moustaches in order to raise money for prostate cancer research during Movember. (cshl.edu)
Supporting our team this year are our Mo Sistas, who are helpfully going to be sporting fake moustaches on a daily basis while committing to Movember's brand new MOVE Challenge, by taking part in an Active Campus session (or other form of physical activity) on a daily basis and sharing their exploits over Twitter! (mmusportblog.com)
It's of course safer — and totally fine — for other top - pantheon comedy filmmakers (Adam McKay, David Wain, etc.) to be content with fake moustaches and other broad, borderline - surrealist goofballery. (slantmagazine.com)