1.  thoughtlessness(n = noun.attribute) inconsiderateness, inconsideration - the quality of failing to be considerate of others;
is a kind of unkindness
has particulars: tactlessness, bluntness
Antonym: thoughtfulness
Derived form adjective inconsiderate₂

2.  thoughtlessness(n = noun.attribute) unthoughtfulness - the trait of not thinking carefully before acting;
is a kind of trait
has particulars: foolhardiness, rashness, recklessness, impulsiveness
Derived form adjective thoughtless₁

N  unskillfulness, want of skill, incompetence, incompentency, inability, infelicity, indexterity, inexperience, disqualification, unproficiency, quackery, folly, stupidity indiscretion, thoughtlessness, sabotage, mismanagement, misconduct, impolicy, maladministration, misrule, misgovernment, misapplication, misdirection, misfeasance, petticoat government, absence of rule, rule of thumb, bungling, failure, screw loose: too many cooks, blunder, etourderie gaucherie, act of folly, balourdise, botch, botchery, bad job, sad work, sprat sent out to catch a whale, much ado about nothing, wild- goose chase, bungler, fool, unskillful, inexpert, bungling, awkward, clumsy, unhandy, lubberly, gauche, maladroit, left-handed, heavy-handed, slovenly, slatternly, gawky, adrift, at fault, inapt, unapt, inhabile, untractable, unteachable, giddy, inconsiderate, stupid inactive, incompetent, unqualified, disqualified, ill-qualified, unfit, quackish, raw, green, inexperienced, rusty, out of practice, unaccustomed, unused, untrained &c, uninitiated, unconversant, shiftless, unstatesmanlike, unadvised, ill-advised, misadvised, ill-devised, ill-imagined, ill-judged, ill-contrived, ill-conducted, unguided, misguided, misconducted, foolish, wild, infelicitous, penny wise and pound foolish, one's fingers being all thumbs, the right hand forgets its cunning, il se noyerait dans une goutte d'eau, incidit in Scyllam qui vult vitare Charybdim, out of the frying pan into the fire, non omnia possumus omnes.

N  rashness, temerity, want of caution, imprudence, indiscretion, overconfidence, presumption, audacity, precipitancy, precipitation, impetuosity, levity, foolhardihood, foolhardiness, heedlessness, thoughtlessness, carelessness, desperation, Quixotism, knight-errantry, fire eating, gaming, gambling, blind bargain, leap in the dark, leap of faith, fool's paradise, too many eggs in one basket, desperado, rashling, madcap, daredevil, Hotspur, fire eater, bully, bravo, Hector, scapegrace, enfant perdu, Don Quixote, knight-errant, Icarus, adventurer, gambler, gamester, dynamitard, boomer, rash, incautious, indiscreet, imprudent, improvident, temerarious, uncalculating, heedless, careless, without ballast, heels over head, head over heels, giddy, wanton, reckless, wild, madcap, desperate, devil-may-care, hot-blooded, hotheaded, hotbrained, headlong, headstrong, breakneck, foolhardy, harebrained, precipitate, impulsive, overconfident, overweening, venturesome, venturous, adventurous, Quixotic, fire eating, cavalier, janty, jaunty, free and easy, off one's guard, post haste, a corps perdu, hand over head, tete baissee, headforemost, happen what may, come what may, neck or nothing, the devil being in one, non semper temeritas est felix, paucis temeritas est bono multis malo.