its "less"
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
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.
Thoughtful.
The cast of A Thoughtless Beauty - 1908 includes: Gertie Potter
Yes, it is. It is the adverb form of the adjective thoughtless and means done in a thoughtless, inconsiderate, or oblivious manner.
kind
The Trials of Thoughtless Thaddeus - 1916 was released on: USA: 12 March 1916
Thoughtless; unsympathetic