Definition from Wiktionary, the free dictionary
frenemy (plural frenemies)
- (humorous) Someone who pretends to be your friend, but is really your enemy.
- 1987, I Ain't No Joke, by Eric B. and Rakim, on the album "Paid in Full": "Another enemy / Not even a frenemy."
- 2001, In France the Seine has all the advantages of Northernness (a quality underrated by our Gallic frenemy) but it is too fatally interested in Paris [...] —John Lanchester, The Debt to Pleasure. 
- 2004, You know when you dump a guy, only to discover years later that he's evolved into the perfect boyfriend—for the high-school frenemy who convinced you to dump him in the first place...? —The Ex-Factor, Andrea Semple. [back cover] 
- 2005, So why did we break up? Enter Blaize St. John, frenemy extraordinaire. She came, she saw, she stole my boyfriend. —Single Girl's Guide to Murder, Joanne Meyer. [back cover] 
- 2007, "Gates made a rare and instructive appearance with his longtime frenemy Steve Jobs." Appeared on Time's June 18, 2007 issue.
- (humorous) A fair-weather friend who is also a rival.