Webster's English Dictionary

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unless

1. un.less \*n-.les, .*n-, in some contexts *n-, *m-, or *n|\ cj [ME 
   unlesse, alter. of onlesse, fr. on + lesse lessB_ 1: under any other 
   circumstance than that : except on the condition that  2: without the 
   accompanying circumstance or condition that : but that :: BUT
2. unless pp : except possibly : EXCEPT