Chiefly Brit., Slang ill-tempered, cross, angry, etc.
Origin of shirtyfrom shirt, as in phrase to have one's (or get someone's) shirt out, to become (or make someone) angry + -y
adjectiveshirt·i·er, shirt·i·est Chiefly British
Ill-tempered; angry: “He saw how shirty she was about it” ( P.G. Wodehouse )
Origin of shirtyProbably from to get someone's shirt out to annoy, or to keep one's shirt on to keep from being annoyed
(comparative shirtier, superlative shirtiest)