Green’s Dictionary of Slang

any form of roughly made or improvised headgear.

1609		Dekker Gul’s Horne-Booke 17: Euery head, when it stood bare or uncouered, lookt like a butter-boxes nowle, having his thrumbd cap on.
1690		Dryden Don Sebastian 17: Hold my dear Thrum-cap: I obey thee cheerfully.
1699		N. Ward London Spy III 69: We fell into a throng of strait-lac’d Monsters in Fur, and Thrum-Caps.
1709		N. Ward Compleat and Humorous Account of Remarkable Clubs (1756) 232: With a thrum Cap upon his Head, a Pair of Mittings upon his Hands, and a Seaman’s Handkerchief about his Neck.
1772		Bridges Burlesque Homer (3rd edn) 10: Smite my thrum-cap, and noddle too.
1792		Kentish Gaz. 2 Oct. 2/3: The hair dishevelled and dirty, ornamented with a thrum-cap tied round with a red garter.
1839		(con. 1703) W.H. Ainsworth Jack Sheppard (1917) 22: His head [was thrust] into a thrum-cap.
1884		Galveston Dly News 28 Aug. 5/6: A fourth with a wooden leg and but one eye [...] with a thrum-cap upon his head.