Heedless, a. — Webster enters it under “heedless”
Without heed or care; inattentive; careless; thoughtless; unobservant. O, negligent and heedless discipline! Shak. The heedless lover does not know Whose eyes they are that wound him so. Waller. -- Heed"less*ly, adv. -- Heed"less*ness, n.
Shades of meaning
Fernald has no entry of its own for heedlessness; he lists it under neglect, temerity.
Where he writes about it:
Venturesomeness dallies on the edge of danger and experiments with it; foolhardiness rushes in for want of sense, heedlessness for want of attention, rashness for want of reflection, recklessness from disregard of consequences. — from temerity
Synonyms
neglecttemerityaudacitycarelessnessdefaultdisregarddisrespectfailurefoolhardinesshardihoodhastinessinadvertenceinattentionindifferenceneglectfulnessnegligenceomissionover-confidenceoversightprecipitancyprecipitationpresumptionrashnessrecklessnessremissnessscornslacknessslightthoughtlessnessventuresomeness