Source of the words "plaid" and "trousers" (4)

I believe the answer is:

erse

(Other definitions for erse that I've seen before include "Old Celtic language" , "Old language spoken by strange seer" , "Irish Gaelic language" , "Old Scots or Irish Gaelic" , "Old name for Celtic languages" .)

I've seen this clue in The New York Times.

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