The word 'unless' originates from the late Middle English phrase 'on lesse that', where 'on' meant 'under' or 'subject to', and 'lesse' was the comparative form of 'little'. The phrase literally meant 'on a less condition than', used to introduce an exception. Over the 15th and 16th centuries, this phrase underwent grammaticalization, fusing into the single word 'unles', and later 'unless'. This evolution transformed a concrete conditional phrase into a concise subordinating conjunction, shifting from a literal sense of 'under a lesser circumstance' to the modern abstract function of introducing the sole condition that would prevent a stated outcome.

The word 'unless' is a fused form derived from the Middle English phrase 'on lesse that' or 'on lesse than', meaning 'on a less condition than'. It is not productively divisible into separate, meaningful morphemes in modern English spelling.