Green’s Dictionary of Slang

the thumb and four fingers, used to smooth one’s hair.

[1653		Urquhart (trans.) Rabelais I xxi: Afterwards he combed his hair with an alman comb, which is the four fingers and the thumb].
c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Welch Comb, the Thumb & four fingers.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn).
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.