"with penetrating understanding into character or hidden nature," 1881, from insight + -ful. Some earlier words in the same sense were insighted (c. 1600), inseeing "having insight" (1590s). Related: Insightfully; insightfulness.
c. 1200, innsihht, "sight with the 'eyes' of the mind, mental vision, understanding from within," from in (prep.) + sight (n.). But the meaning often seems to be felt as "sight into" (something else), and so the sense shifted to "penetrating understanding into character or hidden nature" (1580s). Similar formation in Dutch inzigt, German einsicht, Danish indsigt.
word-forming element attached to nouns (and in modern English to verb stems) and meaning "full of, having, characterized by," also "amount or volume contained" (handful, bellyful); from Old English -full, -ful, which is full (adj.) become a suffix by being coalesced with a preceding noun, but originally a separate word. Cognate with German -voll, Old Norse -fullr, Danish -fuld. Most English -ful adjectives at one time or another had both passive ("full of x") and active ("causing x; full of occasion for x") senses.
It is rare in Old English and Middle English, where full was much more commonly attached at the head of a word (for example Old English fulbrecan "to violate," fulslean "to kill outright," fulripod "mature;" Middle English had ful-comen "attain (a state), realize (a truth)," ful-lasting "durability," ful-thriven "complete, perfect," etc.).
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Harper, D. (n.d.). Etymology of insightful. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/insightful