"square stone for building or paving," mid-14c., from Old French aisseler, Medieval Latin arsella "a little board or shingle," diminutive of Latin assis "a board, plank," also spelled axis, which is perhaps not the same axis that means "axle." De Vaan regards the spelling axis as a hyper-correction. The stone sense is peculiar to English. Meaning "thin slab of stone used as facing on a wall" is from 1823.
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