I can't believe this never ocurred to me until this weekend, but it finally did after hanging out with a friend of a friend who, while being someone who likes to do fun things, has a pretty weak sense of humour. She's no-one I would dare call funny. She seems to find it easier to state things are funny rather than actually laugh at them. But she likes to have fun, so shouldn't that make her fun-ny?
See, if your nose runs, it is "runny", when the sun does it's thing, it is "sunny", and if a bun was at its peak, you might say it was "bunny". So why not "funny" for the fun? How odd it would be to come off an activity suggested by your humourless friend and say, "I'm so glad you suggested that hot air balloon ride! You're SO funny!". I wonder if the word funny arose from people making people laugh and creating a fun time and eventually became used exclusively for the hilarious. Dullards who know how to have fun are now devoid of an appropriate adjective!
Funny, but not fun.