Green’s Dictionary of Slang

(US) a bumper fitted to a customized car.

1949		Hot Rod Mag. Nov. 32/3: Can you fellows tell me what the dickens a ‘nerfing bar’ is? Some of your hot rod slang is rather crude. Robert J. Whittier. Stoughton, Massachusetts. A nerfing bar is a bumper-like affair used on the front of track jobs as shown above (arrow). Some are fancy, some plain. The one shown above is average. -ED.
1958		San Bernardino Co. Sun 11 Oct. 31/3: Handmade nerf bar bumpers were added to both the front and rear.
1965		San Antonio Exp. (TX) 28 Aug. 38/9: [advert] Auto Parts: [...] Sticks, nerf bar, bucket seats.
1974		Dly Sentinel & Leominster Enterprise (MA) 16 July 7/3: Bouchard and Stevens ran nerf bar to nerf bar for several laps.
1986		Age (Melbourne) 26 July 117/4: [advert] Missan 1982 [...] front bull bar and rear tow nerf bar.
1987		Reno Gaz-Jrnl (NV) 19 Nov. 49/4: [advert] Smitty built bumpers, roll bar, nerf bar.
1993		Berlleville Telescope (KS) 12 Aug. 4/5: Jones had a nerf bar falling off the right rear of his car.
2003		Indianapolis Star (IN) 28 Aug. 19/6: A member [of the Diablos biker gang] in California would take 11-pound packages of methamphetamine and place it [sic] in a ‘nerf bar’ accessory found on the bottom of trucks.