Green’s Dictionary of Slang

a euph. for damn n.; thus by dash!

1854		Thackeray Newcomes I 63: If I hadn’t been in India, by dash – he should have been black-balled twenty times over, by dash [Ibid.] Dashed little prig [...] why the dash did they ever let him in here?
1876		B. Harte Gabriel Conroy III 175: Dash it all! what in dash are you dashingly doing here, dash you?
1895		J.L. Williams Princeton Stories 11: Dash it! I wish to dash you fellows would dash quickly get to dash out of here.