Green’s Dictionary of Slang

(US) a fixed-wheel bicycle.

2004		Arizona Dly Star (Tucson, AZ) 10 Jan. E1/1: No gears means no shifting, but riders of fixies can never just stop pedaling.
2015		P. Beatty Sellout (2016) 28: Grown men slowly pedal dirt bikes and fixies through streets clogged with gaggles and coveys of every type of farm bird.
2016		Times 18 Aug. 🌐 This is now a resolutely European nation, with its baristas, flea markets, bearded guys on fixies, and among the fastest broadband speeds in the EU.