If his Wikipedia bio can be believed - he had a tragic childhood in Nazi Germany, was a chess hustler and acquaintance of Albert Einstein. He was also the voice of Gollum in two animated Hobbit movies.
He started doing monologues in the '50s, and was a frequent guest of Merv Griffin (who gave him the "Brother" nickname), Joey Bishopand the Tonight Show. His appearances on Late Night with David Letterman in the '80s were personal favorites of mine.
He died in early 2001. His headstone reads - in part- "As long as there is death, there is hope."
In today's clip from David Letterman's Late Night - check out Theodore's answer to the question, "Did you have a good Summer?"
Bonus Clip: One of my all-time favorite comebacks by Letterman following a particularly insane rant from Theodore.