Green’s Dictionary of Slang

1. the vagina; thus generic for an attractive woman.

1993		T. Winton Lockie Leonard: Scumbuster (1995) 108: I hear she’s a hot grommet.
2003		McGill Reed Dict. of N.Z. Sl. 93: grommet/grummet/grummit Desirable female, from Australian surfie slang for a groupie, evolved from nautical slang for the vagina, by association from the rubber grommet or washer used to screw nuts tight. A shepherd’s grummet is a sheep.

2. (Aus.) the anus.

1945

R.S. Close Love me Sailor 220: Captain [...] take your rutting ship . . . stick it up your grommet!

3. sexual intercourse.

1980

Maledicta IV:2 (Winter) 197: She is a Lady Hotbot, who likes her oats, greens or her grommet.