Green’s Dictionary of Slang

grummet n.

also grummit
[SE grummet, a ring of rope, a washer]

1. (also grummit) the vagina.

1838
1840185018601870188018901900
a.1904
[UK] ‘The Cadger’s Christening’ in Sparkling Songster 21: Peg Pullcock did loudly declare, / That she had the largest old grummit.
[UK]‘Walter’ My Secret Life (1966) I 114: ‘You have been washing your grummet?’ No reply. [Ibid.] XI 2264: The other lad suddenly made a dig at the fair haired one’s clothes outside her grummit.
[UK]Farmer & Henley Sl. and Its Analogues.

2. sexual intercourse.

[UK]Partridge DSUE (8th edn) : since mid-C.19.

3. (orig. N.Z. surfing, also grummit) a woman, esp. as a sex object.

Morrieson Scarecrow 146: A sexy little piece like you is wasted on these hoboes [...]. Just wait till the boys see the class bit of grummit I’ve picked up for myself in the sticks.

4. (N.Z. juv., also grommet) someone or something disliked.

1984
198419851986198719881989
1990
[NZ]informant in DNZE (1998).
Dominion Sun. Times (Wellington) 22 Apr. 4: If you’re called [by adolescents] a grommet (a surfing term)...you’ve been insulted.

In phrases

get one’s grummit (v.)

of a woman, to have sexual intercourse.

[UK]‘Ramrod’ Nocturnal Meeting 56: You have been luckier than the rest of us [...] and got your grummit from him without risk.