Humorous

Humorous is spelt thus, not humourous, even in British English where "humour" has two 'u's.

• Beware the homophone humerus. This is the name of the upper bone in a human arm (and equivalent structures in other species).
  It runs from the shoulder to the elbow.
  
  The common name funny bone, for the ulna (one of the two bones of the lower arm), known for the strange feelings it engenders when it is hit, is partly a pun with humorous/humerus - although the actual bone involved is not the humerus. (Humerus and humorous form one of the sets of homophones listed by the then Poet Laureate Robert Bridges.
  (For more, see Bridges homophones). AWE has a category listing our articles on each of these.)

This is one of the 117 mis-spellings listed as 'Common difficulties' in the section on 'Spelling' within 'Writing' in UEfAP.