Chiefly Brit., Slang ill-tempered, cross, angry, etc.
Origin of shirty; from 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)