Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
unthinkable, thoughtless, unthinking (adjs.)
Anything unthinkable is “inconceivable, impossible, too much”: That we wouldn’t attend was unthinkable. Unthinking means “without thought,” but more often it is a synonym of thoughtless, meaning “heedless or inconsiderate,” as in His unthinking [thoughtless] remarks hurt her feelings. All three words are Standard in these uses.
