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.

"small harpsichord," 1520s, evidently from virgin, but the connection is unclear, unless it means "an instrument played by girls."

Latin, "unless," occurring in legal and administrative phrases used in English, from ni "not " + si "if."

1640s, "subordinate," from Latin servientem "subordinate," present participle of servire "be a servant, be a slave" (see serve (v.)). Compare sergeant. A 17th century word now rare or obsolete unless perhaps in legal phrases.

1580s, from success + -ful. Originally "having or resulting in any kind of success;" since late 19c. it has tended to mean "wealthy, resulting in financial prosperity" unless otherwise indicated. Related: Successfully.

also before-hand, "in anticipation," early 13c., from before + hand, which here is of uncertain signification, unless the original notion is payment in advance or something done before another's hand does it. Hyphenated from 18c.; one word from 19c.

late 14c., hampren "surround, imprison, confine; pack in a container; impede in motion or progress," of uncertain origin; probably from hamper (n.1), unless it is somehow connected to Middle English hamelian "to maim." Related: Hampered; hampering.

mid-15c., stote, "the ermine, especially in its brown summer coat," of uncertain origin. The word bears resemblance to Old Norse stutr "bull," Swedish stut "bull," Danish stud "ox," but the sense is difficult unless a common notion is "male animal."

1770, earlier chirurgical (early 15c.), from surgery + -ical. Related: Surgically.

surgical strike: There is no such thing. Don't use unless in a quote, then question what that means. [Isaac Cubillos, "Military Reporters Stylebook and Reference Guide," 2010]