Someone who pretends to be your friend, but is really your enemy.(noun)
A fair-weather friend who is also a rival.(noun)
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Use "frenemy" in a sentence
"The term frenemy appeared in print as early as 1953, but gained huge popularity in the past few years."
"Though the term frenemy is not gender-specific, there is something troubling about the fact that it is overwhelmingly applied to women."
"The term frenemy, seamlessly blending the words fri (end) and enemy, refers to someone who pretends to be a friend but actually is an enemy --- a proverbial wolf in sheep's clothing in the world of friendships."