The Collaborative International Dictionary of English v.0.48:

Inobservant \In`ob*serv"ant\, a. [L. inobservans. See In- not,
   and Observant.]
   Not observant; regardless; heedless. --Bp. Hurd. --
   In`ob*serv"ant*ly, adv.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "inobservant":
   careless, disregardant, disregardful, distracted, distrait,
   heedless, inadvertent, inattentive, inconstant, incurious,
   indifferent, negligent, nonadherent, noncompliant, nonconforming,
   nonobservant, regardless, thoughtless, uncompliant, unconforming,
   unfaithful, unheedful, unheeding, unloyal, unmarking, unmindful,
   unnoticing, unnoting, unobservant, unobserving, unperceiving,
   unremarking, untrue, unwatchful