>an etymological relationship between the word "thwart" meaning the transverse plank in a skiff on which one sits, and "twat" a vulgar word referring to the vulva

Some of the previous posts were correct with respects to Thwart (athwart), but TWAT came from and entirely different source.


TWAT
[Of obscure origin.]
1. (See quot. 1727.)
Erroneously used (after quot. 1660) by Browning Pippa Passes iv. ii. 96 under the impression that it denoted some part of a nun's attire.
1656 R. Fletcher tr. Martial ii. xliv. 104. 1660 Vanity of Vanities 50 They talk't of his having a Cardinalls Hat, They'd send him as soon an Old Nuns Twat. a1704 T. Brown Sober Slip in Dark Wks. 1711 IV. 182 A dang'rous Street, Where Stones and Twaits in frosty Winters meet. 1719 D'Urfey Pills III. 307. 1727 Bailey vol. II, Twat, pudendum muliebre. Twat-scowerer, a Surgeon or Doctor. E. Ward. 1919 E. E. Cummings Let. 18 Aug. (1969) 61 On Tuesday an Uhlan To her twat put his tool in. 1934 H. Miller Tropic of Cancer 55 A man with something between his legs that could+make her grab that bushy twat of hers with both hands and rub it joyfully. 1959 N. Mailer Advts. for Myself (1961) 101 The clothes off, the guards are driving them into the other room, and smack their hands on skinny flesh and bony flesh, it's bag a tittie and snatch a twot. 1970 G. Greer Female Eunuch 39 No woman wants to find out that she has a twat like a horse-collar. 1973 P. White Eye of Storm iii. 137 This young thing with the swinging hair and partially revealed twat.

2. A term of vulgar abuse. Cf. twit n.1 2b and cunt 2.
1929 F. Manning Middle Parts of Fortune II. xv. 383 Yes, they let a bloody twat like 'im off. 1933 M. Lowry Ultramarine i. 16 He can't help it if you're just a bloody, senseless twat. 1958 H. Williamson Love & Loveless i. 27 Looked a proper twott to me. 1969 P. Roth Portnoy's Complaint 211 Here comes another dumb and stupid remark out of that brainless twat. 1978 J. Updike Coup (1979) iii. 123 Divorce me and you'll have a slot for this new twat, what's her name. 1979 R. Fiennes Hell on Ice ix. 134 Sterns not prows, you twot.

3. U.S. dial. The buttocks.
1950 Publ. Amer. Dial. Soc. xiii. 20 Twat,+the buttocks. 1964 M. Kelly March to Gallows xii. 132, I could tell her what to do with her twat if she's frightened to sit on it.

Thwart n.
[app. a n. use (which came in after 1725) of thwart adv. and adj., having reference to the position of the rowing benches or seats athwart or across the boat. Whether its use was partly due to similarity of sound to thaught, thawt, or thought, previously applied to the same thing, is uncertain. Our latest contemporary instance of ‘thaught or thought’ is of 1721, of thoat 1697, of thout 1725, while our first of ‘thaughts or thwarts’ is of 1736, so that the appellations were continuous in use, as if the one had passed into the other. But, for the full determination of the relations between thoft, thought or thaught, and thwart, fuller evidence between 1500 and 1700 is needed. Cf. thoft, thought2.]
A seat across a boat, on which the rower sits; a rower's bench.
[1721 Bailey, Thoughts, the Rowers Seats in a Boat.] 1736 I (folio), Thaughts, v. Thwarts. Ibid., Thwarts, (a Sea Term) the boards or benches laid a-cross boats and gallies, upon which the rowers sit. 1770 Cook Voy. round World ii. x. (1773) 462 A considerable number of thwarts were laid from gunwale to gunwale. 1776 Falconer's Dict. Marine, Thwart, the seat or bench of a boat whereon the rowers sit to manage the oars. 1897 F. T. Bullen Cruise Cachalot 41 We drew each man his oar across the boat and lashed it firmly down with a piece of line spliced to each thwart.