The Collaborative International Dictionary of English v.0.48:

High-strung \High"-strung`\, a.
   Strung to a high pitch; spirited; sensitive; as, a
   high-strung horse.
   [1913 Webster]

WordNet (r) 3.0 (2006):

high-strung
    adj 1: being in a tense state [syn: edgy, high-strung,
           highly strung, jittery, jumpy, nervy,
           overstrung, restive, uptight]