When the word illusion was borrowed in the mid-fourteenth century from Old French, it meant "scorning" or "derision". Over time, this grew to be less negative and more associated with sensory confusion. The word comes from Latin illusionem, which meant "mocking" or "jeering". That traces to illudere, a verb for "mock" that literally meant "to play with". It contains the prefix in-, meaning "at" or "upon", and the main part is ludere, "to play". This also shows up in words like prelude, ludicrous, and collusion - but those are stories for another time. Ludere comes from the Proto-Indo-European reconstruction leyd, also meaning "to play". Since it was popularized in the early eighteenth century, illusion has been increasing in usage, with peaking in the year 1971.
Hello! I'm Adam Aleksic, a junior studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy trivia, politics, vexillology, geography, board games, conlanging, art history, and law.