The Collaborative International Dictionary of English v.0.48:

Overzealous \O"ver*zeal"ous\, a.
   Too zealous.
   [1913 Webster]

WordNet (r) 3.0 (2006):

overzealous
    adj 1: marked by excessive enthusiasm for and intense devotion
           to a cause or idea; "rabid isolationist" [syn: fanatic,
           fanatical, overzealous, rabid]

Moby Thesaurus II by Grady Ward, 1.0:

88 Moby Thesaurus words for "overzealous":
   accident-prone, at fever pitch, balking, balky, bigoted, breakneck,
   bulldogged, bulletheaded, bullheaded, careless, case-hardened,
   delirious, desperate, devil-may-care, dogged, dogmatic,
   extravagant, extreme, extremist, fanatic, fanatical, febrile,
   feverish, frantic, frenetic, frenzied, furious, haggard,
   hardheaded, harum-scarum, hasty, headlong, headstrong, hectic,
   hotheaded, hurried, hysteric, impetuous, infatuated, inordinate,
   insane, intolerant, irrational, mad, mulish, obstinate,
   opinionated, overanxious, overdesirous, overdevout, overeager,
   overenthusiastic, overreligious, overrighteous, perfervid,
   persevering, pertinacious, pigheaded, precipitant, precipitate,
   precipitous, rabid, reckless, restive, sanctimonious, self-willed,
   set, slap-bang, slapdash, stiff-necked, strong-willed,
   strongheaded, stubborn, sulky, sullen, tenacious, ultrareligious,
   ultrazealous, uncooperative, unreasonable, unregenerate, wanton,
   wild, wild-eyed, wild-looking, willful, zealotic, zealous