bobstay, n. — Green’s Dictionary of Slang

bobstay n.

[naut. jargon bobstay, a rope that holds down the bowsprit of a ship, counteracting the upward force of the fore-mast stays]

the frenum or ligament of the penis.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Bob Stay. a Rope which holds the Bowsprit to the stem or Cutwater. figuratively, the Frenum of the Yard.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn).
1805		J. Davis Post Captain (1813) 74: Come, Cassandra [...] let us descend and turn in. If I don’t ease my laniard I shall carry away my bob-stay.
1811		Lex. Balatronicum.
1984		(con. WW2) T. Jones Heart of Oak [ebook] There I was to learn [...] doctor’s chum, and keep my bobstay and shit-kickers clean [etc.].