happy

\ \ [14] The Old and Middle English word for ‘happy’ was what in modern English has become silly. This began to change its meaning around the 15th century, and obviously an opportunity began to open up for an adjective expressing ‘contentment’ (as opposed to positive ‘joy’, denoted then by glad, fain, and joyful).

\ \ The gap was partly filled by a weakening in the meaning of glad, but waiting in the wings was happy, a derivative of the noun hap ‘chance, luck’ (source of happen), which when it was coined in the 14th century meant ‘lucky, fortunate, prosperous’. The main modern sense ‘highly pleased or contented’ developed in the early 16th century.

\ \ Cf.⇒ HAPPEN