Green’s Dictionary of Slang

a general excl. of annoyance, surprise etc (esp. attrib. to a Welsh speaker); a euph. for ‘God’s oath!’ (cf. cotzooks! ).

1671		H. Crouch The Welch Traveller line 247: The quean she s--- in her hand and cast it in hur face: Cuts plutternails! beshrew her heart!
1685–88		‘The Sorrowful Citizen’ in Ebsworth Roxburghe Ballads (1891) VII:2 280: ‘Cot-zo!’ quoth the maiden, ‘pray what have we here?’.
1690		‘Teague’ Teagueland Jests II 167: Cots plut (says the Welshman) hur has not hur Cun pounce one, two, threee times.
1699		‘Song’ in Playford Pills to Purge Melancholy I 311: Her was the prettyest Fellow / At Foot-ball, or at Crickett; / At Hunting Chace. or nimble Race, / Cots-plut how her cou’d prick it.
c.1775		Hist. of Jack and the Giants Pt I 16: The Welch Giant seeing, cried out, Cots plut.
1825		C.M. Westmacott Eng. Spy I 131: ‘Cot’s flesh!’ cries Morgan, ‘cease your mockings.’.