Green’s Dictionary of Slang

1. a hairstyle, popular c.1860–70, whereby middle-class women wore their hair in a net, which then fell onto their shoulders.

1909

(ref. to 1860) J. Ware Passing Eng. of the Victorian Era 23/2: Beaver-tail (Mid.-class, 1860). A feminine mode of wearing the back-hair, turned up loose in a fine thread net (called ‘invisible’) which fell well on to the shoulders. When the net is now worn, generally by lazy girls of the people, it is fixed above the neck.

2. (US) a cosh, a sap.

1970		J. Wambaugh New Centurions 199: [L]ifting his shirt and showing Roy the huge black beaver-tail sap he carried .
1972		J. Wambaugh Blue Knight 242: I put my gun in my holster, reached for my beavertail, and sapped him across the left collarbone.
1983		M. Helprin ‘White Horse and Dark Horse’ in Winter’s Tale 521: A set of two derringers [...] brass knuckles with projecting stilettos, spiked beaver tails, blackjacks.
2023		(con. 1962) J. Ellroy Enchanters 100: I grabbed my [...] beavertail sap.