Green’s Dictionary of Slang

the head, esp. in phr. tuck in your tuppenny.

1835		G. Kent Modern Flash Dict. 19: Jemmy – twopenny, head.
1849		Era (London) 2 Sept. 5/2: He tucked in his twopenny, squared his toes and elbows.
1859, 1860		Hotten Dict. of Modern Sl. etc.
1861		Bell’s Life in Sydney 4 May 3/4: The undermentioned young gentlemen [...] ‘ducked their twopennies’ to the Water Police magistrate on Tuesday.
1864, 1867, 1870		Hotten Sl. Dict.
1866		Letters by an Odd Boy 27: Tuck in your twopenny, and let me have an over before I begin!
1884		Ally Sloper’s Half Holiday 14 June 53: [caption] Tuck in yer Tuppeny [sic].
1899		A. Binstead Gal’s Gossip 104: After I had ducked my tuppenny to allow him to slip the neckband under my rear stud.
1933		‘George Orwell’ Down and Out in Complete Works (1986) I 178: [note] ‘Twopenny’ is arrived at like this: loaf of bread – twopenny loaf – twopenny.