The compound adjective 'thought-out' is a relatively modern English formation, emerging from the combination of two native Germanic elements. The first part, 'thought', originates from the Old English noun for the process of thinking, itself derived from the Proto-Germanic root thankō-. The second part, 'out', is a fundamental Old English adverb from Proto-Germanic ūt, signifying movement away or to a conclusion. The logic of the compound is transparently metaphorical: just as something physically extended or fully brought out is complete, an idea that has been fully 'thought out' has been developed to its logical conclusion. This participial construction, where a past participle is formed from a phrasal verb ('to think out'), follows a productive pattern in English for creating adjectives that describe a state resulting from a completed action, emphasizing thoroughness and deliberate planning.

Structure: thought + out - thought: From Old English 'þōht', a noun of action from the verb 'þencan' (to think). In this compound, it functions as the nominal element representing the process or result of thinking. - out: From Old English 'ūt', an adverb meaning 'outside, away, forth'. In this compound, it functions as an adverbial particle indicating completion, thoroughness, or extension.