Green’s Dictionary of Slang

tunnel n.

1. the vagina.

[UK]‘The Female Tunnel’ in Funny Songster in Spedding & Watt (eds) Bawdy Songbooks (2011) III 49: Pat Grummet was the chosen man, as large a man as Brunell [sic], / Who had no doubt he would succeed in boring of her tunnel.

2. (US) the mouth.

[US]Sepe & Telano Cop Team 78: Shut your tunnel and get in the car.

3. (US gay) the anus.

[US]B. Rodgers Queens’ Vernacular 19: the rectal opening, anus [...] tunnel (‘Divinity student or not, he had it right up my tunnel’).

4. (Aus. prison) a woman.

[Aus]Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Tunnel. Woman.

SE in slang uses

In compounds

tunnel rat (n.) [note Vietnam era milit. tunnel rat, a soldier who entered – and fought in – the extensive enemy tunnel system]

1. (US prison) one who escapes by digging their way out of prison.

[US]Bentley & Corbett Prison Sl. 109: Tunnel Rat also To Tunnel […] and the escaping inmates are known as tunnel rats. (Archaic: dig a hole).

2. (US, N.Y.) a transit police officer.

[US]M. McAlary Crack War (1991) 14: In July the transit force grabbed him, making Eddie a tunnel rat.
tunnel stiff (n.) [SE tunnel + stiff n.1 (5e)]

(US) an underground tunnel worker.

[US]L.A. Times 22 Mar. 🌐 Those attracted to the work — known as ‘sandhogs’ in the East; ‘tunnel stiffs’ in the West — are, by reputation, hard-living boomers who travel from job to job and are somewhat casual about risks. ‘You finally get to where you don’t pay much attention,’ said Audrain Weatherl of Sacramento, a veteran tunnel stiff and an official of the Laborers International Union.

In phrases

tunnel (of love) (n.)

the vagina.

[UK]Farmer & Henley Sl. and Its Analogues.
[UK]Farmer Vocabula Amatoria (1966) 170: Labyrinthe, m. The female pudendum; ‘the tunnel.’.
[US]‘Troy Conway’ Cunning Linguist (1973) 21: My last view of her [...] was to see her popping black eyes, her great rolling breasts and her ever-widening Tunnel of Love opening to receive the bomb load.
[US]Maledicta IV:2 (Winter) 182: Occasionally, the colloquialism acknowledges the functions of the part, as is seen with [...] tunnel of love.
[UK](con. 1953) A. Wheatle Island Songs (2006) 117: Sweet girl inna de pink frock! Yuh waan to rinse an’ clean me bamboo inna ya tunnel ah love?
Skins ser.1 ep.1 [TV script] Sid’s flying solo down the tunnel of love and he needs support.