Synonym for thoughtless is "stupid"

blind [adjective]

means "characteristic"
unenlightened, thoughtless

careless [adjective]

means "unconsidered"
unthinking, inconsiderate, negligent, superficial, unmindful, thoughtless

foolhardy [adjective]

means "careless"
thoughtless

hasty [adjective]

means "careless"
thoughtless

heedless [adjective]

means "blind"
thoughtless

hotheaded [adjective]

means "fiery"
thoughtless, incautious

ill-advised [adjective]

means "brash"
foolish, inappropriate, rash, reckless, thoughtless, unwise

improvident [adjective]

means "imprudent"
thoughtless, careless, heedless, unwary, incautious

inadvertent [adjective]

means "unconsidered"
unthinking, inconsiderate, negligent, superficial, unmindful, thoughtless

incautious [adjective]

means "rash"
hasty, reckless, thoughtless, hot-headed

inconsiderate [adjective]

means "unconsidered"
unthinking, careless, negligent, superficial, unmindful, thoughtless

listless [adjective]

means "apathetic"
indifferent, unconcerned, passive, heedless, thoughtless, careless

mechanical [adjective]

means "perfunctory"
regular, thoughtless, involuntary, artificial, habitual, unreasoning

perfunctory [adjective]

means "mechanical"
regular, thoughtless, involuntary, artificial, habitual, unreasoning

remiss [adjective]

means "negligent"
careless, heedless, inattentive, lax, neglectful, thoughtless

shabby [adjective]

means "behaviour"
unkind, contemptible, low, thoughtless, unjust, mean

short [adjective]

means "behaviour"
curt, abrupt, sharp, thoughtless, inconsiderate, rude

short-sighted [adjective]

means "myopic"
astigmatic, heedless, blind, unthinking, thoughtless, reckless

shortsighted [adjective]

means "improvident"
heedless, unthinking, thoughtless, reckless, unmindful, lacking foresight

slapdash [adjective]

means "slipshod"
haphazard, messy, untidy, sloppy, thoughtless, disorderly

unconsidered [adjective]

means "careless"
unthinking, inconsiderate, negligent, superficial, unmindful, thoughtless

unguarded [adjective]

means "careless"
imprudent, incautious, unwary, thoughtless

unwary [adjective]

means "imprudent"
thoughtless, careless, heedless, improvident, incautious

thoughtlessness [noun]

means "unsteadiness"
flightiness, nonsense, fickleness, foolishness, giddiness, rashness, folly

fickleness [noun]

means "unsteadiness"
flightiness, thoughtlessness, nonsense, foolishness, giddiness, rashness

flightiness [noun]

means "unsteadiness"
nonsense, thoughtlessness, fickleness, foolishness, giddiness, rashness

folly [noun]

means "unsteadiness"
flightiness, thoughtlessness, fickleness, foolishness, giddiness, rashness

foolishness [noun]

means "unsteadiness"
flightiness, thoughtlessness, fickleness, nonsense, giddiness, rashness

nonsense [noun]