Green’s Dictionary of Slang

letch n.

1. a strong sexual desire; a yearning.

1796		Grose Classical Dict. of the Vulgar Tongue (3rd edn) n.p.: Letch. A whim of the amorous kind, out of the common way.
1811		Lex. Balatronicum [as cit. 1796].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1796].
1880		‘Lady Pokingham’ in Pearl 10 Apr. 28: He had a perfect plethora of disgusting leches, which he required to be enacted before he could experience sensual excitement.
1888–94		‘Walter’ My Secret Life (1966) I 108: She had taken a letch for me, or else, being hot cunted [...] dearly liked my poking her.
1930		J.P. McEvoy Hollywood Girl 15: I told him I was off the legit for a while and had a big letch for the movies.
1953		S. Bellow Augie March (1996) 113: Her pince-nez takes you in, and you don’t know what a letch she’s got.
1955		T. Williams Cat on a Hot Tin Roof Act I: What makes you think that Big Daddy has a lech for you, Maggie?
1970		San Diego Sailor 41: I hadn’t let myself build up anything beyond a good letch with the kid. [Ibid.] 56: I liked his insatiable lech for me.

2. (orig. US) a lecher.

1943		H.A. Smith Life in a Putty Knife Factory (1948) 123: If anybody noticed what I was doing, they’d think I was an old letch.
1964		L. Nkosi Rhythm of Violence II ii: The old lech! he would love that!
1972		C. Bukowski Erections, Ejaculations etc. 111: Listen. Bukowski, you old letch. . .
1974		R. Stone Dog Soldiers (1976) 70: Lerner [...] is a senile Vietnamese asshole. And he’s a lech.
1982		W. Wharton Midnight Clear 93: Maybe it’ll get Father’s mind off how big a lech he is.
1989		C. Hiaasen Skin Tight 216: He was not the booze-swilling lech he had appeared to be at the nudie joint.
1999		Guardian Rev. 12 June 8: The gentleman publisher turned into alcoholic lech by the evils of big business.
2002		S. Maloney Something Fishy (2006) 157: Tony was a tosser and a letch [...] but those aren’t capital offences.
2004		J. Ellroy ‘Hot-Prowl Rape-O’ in Destination: Morgue! (2004) 278: The little lech laffed.
2007		S. Maloney Sucked In 140: The old letch cost me my job.
2018		C. Hammer Scrublands [ebook] Sorry to be such a predictable middle-aged lech.
2022		A. Parks May God Forgive 154: ‘For a dirty old lech he wasn’t the worst’.

3. sexual play.

1966

Current Sl. I:3 5/2: Lech, n. Any act of sexual play.

In derivatives

letchy (adj.)

lecherous.

2000

N. Griffiths Grits 423: Colm — being his usual letchy and insensitive self [...] grins and goes — Phwoar. Body on er.

In compounds

letchwater (n.)

1. semen.

1980		Maledicta IV:2 (Winter) 196: There are many terms for the ejaculate […] cream, letchwater, lewd infusion, living injection, fetch, fuck, melted butter, honey, spunk and lather are all fairly common terms.
2023		R. Milward Man-Eating Typewriter 108: ‘Are you pissing, dear sir?’ [...] He promised me it was letch-water.

2. vaginal fluid.

1980

Maledicta IV:2 (Winter) 182: We have recognised the effects of the flow of this letchwater by referring to the lady’s water-box, -course, -gap, -gate [...], -engine, -works or -mill.