Where does “vapourless” come from?

vapourless (English) comes from English less, from Middle English les, from Old English lēas, from Proto-Germanic *laus.

Ancestry of “vapourless”, step by step

vapourless traces back through two separate lines of ancestry.

via English less

StepLanguageWordMeaning
1EnglishlessTo a smaller extent or degree; comparative form...
2Middle Englishlesfalse; lying; deceptive; falsehood; a lie
3Old Englishlēasfalse, lax; false, void, loose; devoid of, loose...
4Proto-Germanic*laus

via English vapour

StepLanguageWordMeaning
1EnglishvapourBritish standard spelling of vapor
Every word from Proto-Germanic *lausEvery word from Old English lēasEvery word from Middle English les