mid-15c., earlier onlesse, from (not) on lesse (than) "(not) on a less compelling condition (than);" see less. The first syllable originally was on, but the quality of negation in the word and the lack of stress changed it to un-. "Except could once be used as a synonym for unless, but the words have now drawn entirely apart" [Century Dictionary].
early 15c. as a shortening of unless. Extended contraction lessen, less'n, U.S. dialectal, is attested from 1881.
Latin, "unless," occurring in legal and administrative phrases used in English, from ni "not " + si "if."
traditional English dance of persons in costume, mid-15c., moreys daunce "Moorish dance," from Flemish mooriske dans, from Old French morois "Moorish, Arab, black," from More "Moor" (see Moor). It is unknown why the English dance (which typically is based on the Robin Hood stories) was called this, unless it is in reference to fantastic dancing or costumes (compare Italian Moresco, a related dance, literally "Moorish;" German moriskentanz, French moresque).