A brand name for a type of insulating board made from wood fiber that's used in construction and building.
Proprietary trademark coined in the early 20th century, possibly from cello- (cell-like structure) + tex (textile-like texture). Became a genericized trademark.
Celotex was such a popular brand name for building insulation that people started calling all similar products 'celotex'—it's how some brand names get so famous they become the regular word for the whole category!
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