Green’s Dictionary of Slang

(orig. US) to swim in the nude; also as adj.; thus skinny-dipping n.

1956		Berkshire (MA) Eagle 6 June 25/3: Once an all-male hideout for skinny-dip swimming, kelly pool and inspirational talks, the YMCA has been coeducational for a number of years, he said.
1964		Albany Ledger (MO) 2 Apr. 9/2: You’ve probably read Mr [Lyndon Baines] Johnson goes skinny dipping [...] Mr Johnson [is] a life-long skinny-dipper.
1966		R.H. Rimmer Harrad Experiment 56: It’s amazingly warm for this time of year. Come on. We can skinny dip.
1977		L. Heinemann Close Quarters (1987) 95: There were the times we skinny-dipped in the river at the laterite pits.
1980		in S. Terkel Amer. Dreams (1982) 396: There was a lot of pot smoking and skinny-dipping.
1985		‘Thommo’ Dict. Aus. Swearing & Sex Sayings 119: SKINNY DIPPING — Nude bathing.
1994		N. McCall Makes Me Wanna Holler (1995) 7: We even ventured to Crystal Lake and skinny-dipped.
2006		G. Iles Turning Angel 26: She could have been skinny-dipping.
2016		Observer (London) 16 Oct. 1/2: Two men went ‘skinny-dipping’ in Bournemouth.
2020		G. Disher Consolation 155: Hirsch idly wondered if the kids would have gone skinny-dipping.