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]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "testy":
   bad-tempered, bearish, bitchy, cankered, cantankerous, captious,
   choleric, churlish, contentious, crabbed, crabby, cranky, cross,
   cross-grained, crotchety, crusty, cussed, disagreeable, edgy,
   excitable, feisty, fractious, fretful, grouchy, grumpy,
   hot-tempered, huffish, huffy, ill-humored, ill-tempered, irascible,
   irritable, mean, on edge, ornery, peevish, perverse, petulant,
   pissed off, quarrelsome, querulous, quick-tempered, short-tempered,
   snappish, spiteful, spleeny, splenetic, tetchy, touchy, ugly,
   waspish