Definition from Wiktionary, the free dictionary
- (idiomatic) A duplicate or counterpart that acts in a contrary, nefarious, or insidious manner.
- As a ploy to discredit the reformist politician, the opposing party had hired a lookalike to act as his evil twin and be seen in a variety of compromising situations.
- 2003: Peter Halstead, Sea Sun 
- ...we see the flip side, the evil twin, the dark side of addition. Human nature contains both a thing and its opposite in varying degrees of tension, and so for every plus there is a minus.
- 2004: Bob Sullivan, Your Evil Twin: Behind the Identity Theft Epidemic 
- ...[he]..lived his life under [another man's name], becoming his evil twin: Staas bought and sold homes, opened bank accounts, obtained credit, married twice, and was arrested at least three times.
- (networking) A rogue wireless access point installed near a legitimate one for purposes of eavesdropping or phishing.