(throws open much-tattered Webster's) Indefatigable: Adjective. Untiring or unwavering. (throws Webster's aside) Like I've said before somewhere, the man must have been a real laugh riot. Had I been there, I'd have been laughing so hard I'd have tears running down my face.
But I'd like to know what kind of response he'd get from the parents. Can you imagine, a parent at the doors to the studio saying to the kids, "All right, I want you to be on your very best behavior for the nice Muppet people, because they're very hard at work..." and then the doors fly open and then there's the indefatigable Mr. H. doing his level best to goof off...
Simply put, Richard was (is) just like the favorite young uncle who makes the kids laugh (and the grandparents gasp with horror) at every family reunion.