Difficult

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late Middle English: back-formation from difficulty.

wiktionary

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From Middle English difficult (ca. 1400), a back-formation from difficultee (whence modern difficulty), from Old French difficulté, from Latin difficultas, from difficul, older form of difficilis(“hard to do, difficult”), from dis- + facilis(“easy”); see difficile. Replaced native Middle English earveþ(“difficult, hard”), from Old English earfoþe(“difficult, laborious, full of hardship”), cognate to German Arbeit(“work”).

etymonline

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difficult (adj.)

c. 1400, "not easy, requiring or dependent on effort; troublesome, arduous," apparently an unetymological back-formation from difficulty. French has difficile, Latin difficilis. Of persons, "hard to please," from 1580s. Related: Difficultly.