The Collaborative International Dictionary of English v.0.48:

Testy \Tes"ty\, a. [Compar. Testier; superl. Testiest.] [OF.
   testu obstinate, headstrong, F. t[^e]tu, fr. OF. teste the
   head, F. t[^e]te. See Test a cupel.]
   Fretful; peevish; petulant; easily irritated.
   [1913 Webster]

         Must I observe you? must I stand and crouch
         Under your testy humor?                  --Shak.
   [1913 Webster]

         I was displeased with myself; I was testy. --Latimer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

testy
    adj 1: easily irritated or annoyed; "an incorrigibly fractious
           young man"; "not the least nettlesome of his countrymen"
           [syn: cranky, fractious, irritable, nettlesome,
           peevish, peckish, pettish, petulant, scratchy,
           testy, tetchy, techy]