The word 'thoughtlessly' is a layered construction built upon the Old English noun 'thought', which itself originates from the Proto-Germanic verb for thinking. The addition of the suffix '-less' transforms the noun into the adjective 'thoughtless', literally meaning 'devoid of thought'. Finally, the adverbial suffix '-ly' is appended to describe the manner of an action. Thus, the etymological logic moves from the core mental faculty (thought), to its absence (thoughtless), and finally to the way an action is performed without that faculty (thoughtlessly). This progression cleanly maps the literal, compositional meaning onto the modern abstract sense of acting without consideration or care.

Structure: thought + less + ly - thought (Old English 'þōht', from 'þencan' meaning to think): The core concept of the mind's activity. - less (Old English '-lēas', meaning devoid of, free from): An adjective-forming suffix indicating absence. - ly (Old English '-līce', forming adverbs from adjectives): An adverb-forming suffix indicating manner.