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The opposite of "thoughtful" with a prefix would be "thoughtless."
Mean, thoughtless, uncaring...
Stumped, thoughtless, stuck, unthinking, unmindful....
I can see why the word thoughtless might seem to be an antonym for pensive, since a pensive person thinks a lot and a thoughtless person doesn't, however, thoughtlessness is really about irresponsibility, it is not really about thought in general. The opposite of thoughtless is thoughtful or responsible, not pensive. The opposite of pensive would be spontaneous, or glib.
Careless, foolish, idiotic, stupid, ignorant, senseless, irrational, thoughtless, uneducated, or reckless. Those words are the opposite of wise.
The opposite of the past tense verb thought (assumed) could be "knew" and the adjective could be "known."The opposite noun for thought (consideration) could be action.
How thoughtless of him to not even give me a tip.
Thoughtless was created on 2002-10-15.
Only a thoughtless person would ask me for a sentence like that.
No, "thoughtless" is an adjective that describes someone who acts without considering the consequences or feelings of others.
Figure it out yourself
The opposite of 'OFF' is 'ON' - -have you never seen those words on each side of a switch