Old English þoht, geþoht "process of thinking, a thought; compassion," from stem of þencan "to conceive of in the mind, consider" (see think). Cognate with the second element in German Gedächtnis "memory," Andacht "attention, devotion," Bedacht "consideration, deliberation."
Bammesberger ("English Etymology") explains that in Germanic -kt- generally shifted to -ht-, and a nasal before -ht- was lost. Proto-Germanic *thankija- added a suffix -t in the past tense. By the first pattern the Germanic form was *thanht-, by the second the Old English was þoht.
Second thought "later consideration" is recorded from 1640s. Thought-crime is from "Nineteen Eighty-Four" (1949); thought police is attested from 1945, originally in reference to war-time Japanese Special Higher Police (Tokubetsu Koto Keisatsu).
word-forming element meaning "lacking, cannot be, does not," from Old English -leas, from leas "free (from), devoid (of), false, feigned," from Proto-Germanic *lausaz (cognates: Dutch -loos, German -los "-less," Old Norse lauss "loose, free, vacant, dissolute," Middle Dutch los, German los "loose, free," Gothic laus "empty, vain"), from PIE root *leu- "to loosen, divide, cut apart." Related to loose and lease.
c. 1200, "contemplative, occupied with thought," from thought + -ful. Also in Middle English, "prudent; moody, anxious." Meaning "showing consideration for others" is from 1851 (compare thoughtless.) Related: Thoughtfully; thoughtfulness.
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Definitions of thoughtless from WordNet
showing lack of careful thought;
the debate turned into thoughtless bickering
without care or thought for others; "the thoughtless saying of a great princess on being informed that the people had no bread; `Let them eat cake'";