Humourless and humoursome are relatively modern coinages within English, humour + living affix.

Humorous however is a Latin word űmôrôsus. The stem is űmôr- with long O. (The initial H arose in post-classical Latin when H was largely silent in common speech.) The long vowel became short in final position, giving űmor 'humour'.

This vowel difference was preserved in Old/Middle French, giving rise to modern French humeur ~ humoreux and English humour ~ humorous.