The Collaborative International Dictionary of English v.0.48:

Inattentive \In`at*ten"tive\, a. [Cf. F. inattentif.]
   Not attentive; not fixing the mind on an object; heedless;
   careless; negligent; regardless; as, an inattentive spectator
   or hearer; an inattentive habit. --I. Watts.

   Syn: Careless; heedless; regardless; thoughtless; negligent;
        remiss; inadvertent. -- In`at*ten"tive*ly, adv. --
        In`at*ten"tive*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

inattentive
    adj 1: showing a lack of attention or care; "inattentive
           students"; "an inattentive babysitter" [ant: attentive]
    2: not showing due care or attention; "inattentive students";
       "an inattentive babysitter"; "neglectful parents" [syn:
       inattentive, neglectful]

Moby Thesaurus II by Grady Ward, 1.0:

101 Moby Thesaurus words for "inattentive":
   apathetic, artless, ataractic, blase, careless, casual,
   culpably negligent, derelict, devil-may-care, disinterested,
   dispassionate, disregardant, disregardful, distracted, distrait,
   distraught, easygoing, half-assed, heedless, inadept, inadvertent,
   inapt, inconstant, incurious, indifferent, inefficient, inept,
   inexcitable, inexpert, inobservant, insouciant, lackadaisical,
   laissez-faire, lax, listless, loose, mediocre, mindless,
   neglectful, neglecting, negligent, nonadherent, nonchalant,
   noncompliant, nonconforming, noninterfering, nonobservant,
   nonrestrictive, off-guard, overly permissive, pedestrian,
   perfunctory, permissive, pococurante, poor, procrastinating,
   reckless, regardless, relaxed, remiss, scamping, skill-less,
   skimping, slack, slighting, slurring, thoughtless, turned-off,
   unanxious, unapt, uncircumspect, uncompliant, unconcerned,
   unconforming, undeft, undexterous, undextrous, undiscriminating,
   unfacile, unfaithful, unguarded, unheedful, unheeding,
   unintelligent, uninterested, unloyal, unmarking, unmindful,
   unnoticing, unnoting, unobservant, unobserving, unperceiving,
   unproficient, unremarking, unrigorous, unskillful, unsolicitous,
   untrue, unwary, unwatchful