Green’s Dictionary of Slang

(US) the penis.

1950		in Randolph & Legman Ozark Folksongs and Folklore (1992) I 453: Pappy tied me to the hitchin’-rack, / Stuck his old tillywhacker up my crack.
1992		(con. 1929) Randolph & Legman Ozark Folksongs and Folklore I 113: Sung as above [...] August 3, 1929. In the chorus, line 1: tilly-whacker sometimes talley-wagger, or dilly-whacker, meaning the penis. [Ibid.] 313: ‘Yaller gal, yaller gal, layin’ on her back, / Massa took his tillywhacker, stuck it up her crack.’ [...] Sung as above by Mr. C.M., Joplin, Missouri, April 3, 1928. [Ibid.] 402: Captain Stan Hugill’s ‘Unprintable’ manuscript supplement [...] ‘Sally’s in the garden shifting sand, / With a fathom of dillywhacker in her hand’.
1992		Randolph & Legman Ozark Folksongs and Folklore I 453: Tilly-whacker, penis, originally British ‘tallywagger’.