Fifteen years ago, Bill Hicks performed a routine on Letterman that never aired. The content wasn’t appropriate — maybe he didn’t do the material he said he was going to do — maybe some censorial types objected. A few months later, Hicks passed away from pancreatic cancer — a condition he’d hidden from all but his closest family members. Last night on Letterman, Dave apologized to Bill’s mother and aired the never-aired segment.