Green’s Dictionary of Slang

aaron’s rod n.

[punning on rod n. (1), f. Num. 17:8: ‘Behold, the rod of Aaron...was budded, and brought forth buds, and bloomed blossoms and yielded almonds’]

the penis.

[Aus]Satirist & Sporting Chron. (Sydney) 25 Feb. 2/3: It is said [...] that Aaron’s Rod that budded, has been at its amusing tricks again [...] and has not only budded but thrown out a shoot .
[UK]‘Three Chums’ in Boudoir III 90: His lips met hers in a long luscious kiss, so exciting that his Aaron’s rod was as stiff as possible.
[UK]Farmer Vocabula Amatoria (1966) 72: Commissaire (le). The penis; ‘Aaron’s rod’.
[US]R.O. Scott Gay Sl. Dict. [Internet] Aaron’s rod: synonym for penis.