Green’s Dictionary of Slang

ambassador (of love) n.

[it ‘presents its credentials’ to the vagina]

(US) the penis.

[UK]Harris’s List of Covent-Garden Ladies 67: [She] is very strict in examining the ambassador of love e’er she receives his tribute.
[US]‘J.M. Hall’ Anecdota Americana I 108: He was about to inject the tip of his feebly erect ambassador.
HPS-Online [Internet] ‘Chinese Sexual glossary’ Penis: Jade Stem, Jade Implement, Yang Peak, Yang Weapon, Turtle Head, The Ambassador.