Green’s Dictionary of Slang

tetchy, cantankerous, likely to take offence.

1810		J. Beresford Bibliosophia 119: And very lucky it was, by the way (considering how very miffy those Ladies are said to have been).
1836		T. Haliburton Clockmaker I 232: Well, says I, I’ll tell you if you won’t be miffy with me.
1878		Notts. Guardian 20 May 10/4: So you call me miffy, do you? What a libel — I am nothing of the kind.
1911		S. Ford Torchy 135: Mr. Robert was a little miffy because I hadn’t [...] played Clifford for a favorite from the start.