Merriam Webster indicates that the word "frenemy" has been in existence since 1977, although it's exact origins are unknown. The term became popular in 2005 when Paris Hilton and Nicole Richie were filming The Simple Life. The two friends had a falling out and Hilton reportedly refused to work with Richie. Richie was let back on the show but had to film separately from Hilton. This switching back and forth between fighting and being friendly prompted some in the media to call them frenemies.
A frenemy is someone who will pretend to be close to you, but at the same time belittle you or say snarky things to make you feel bad.
More about frenemies: