Green’s Dictionary of Slang

truncheon n.

[cit. 1834 is a deliberate double entendre]

1. the penis.

[UK] ‘The Riddle’ in Playford Pills to Purge Melancholy II 72: ’Twas the Truncheon Mars did use, A Bed-ward bit, which Maidens choose.
[UK]N. Ward Secret Hist. of Clubs 378: [The] Sort of Ladies who prefer Mars’s Truncheon to Apollo’s Harp.
[UK] in D’Urfey Pills to Purge Melancholy IV 72: ’Twas the truncheon Mars did use, / A Bed-ward bit which Maidens chuse.
[UK]Cleland Memoirs of a Woman of Pleasure (1985) 63: He drew up his shirt, and bared all his hairy thighs, and stiff staring truncheon, red-topt, and rooted into a thicket of curls.
[UK] ‘Flare Up!’ in Black Joke 4: His truncheon was so stout and long [...] Some scores of them he got with child.
[UK]E. Sellon Phoebe Kissagen 34: [He] let loose his great truncheon, and flung his arms round us both.
[UK] ‘Lady Pokingham’ in Pearl 1 July 24: Her hands nervously unbuttoned Mr. William’s trousers, and grasped his ready truncheon.
[UK]‘Walter’ My Secret Life (1966) IX 1766: I pulled out my red tipped truncheon.
[US]Maledicta IV:2 (Winter) 194: We also find the policeman represented with his truncheon, billie, night stick or copper stick (which is also a housewife’s tool of the last century).

2. an erection.

[Ire]P. Howard Curious Incident of the Dog in the Nightdress 268: I’ve got a focking truncheon on me that could beat Oisinn away from an all-day breakfast buffet.

In phrases

punching the truncheon (n.)

masturbation.

[Aus]G. Seal Lingo 126: tug the slug; beat the meat and punchin’ the truncheon, are all descriptions of male masturbation.