Green’s Dictionary of Slang

apt to take offence for minimal reason.

c.1698		B.E. Dict. Canting Crew n.p.: Mifty [sic] apt to take Pet, or be out of Humour.
1725		New Canting Dict.
1740		Cibber Life of Colley Cibber 248: She mutter’d out her Words in a sort of mifty manner, at my low Opinion of her.