name that funk

so.....good pal and customer walks into the shop with a bike this past week and he wants to drop it off for a spring deluxe tune up (clean,lube and tune). As i'm checking the bike in something kinda catches my eye.....i point out what appears to be a large piece of flesh.....customer shrugs his shoulder and says he didn't notice it, but with one of those sheepish grins that seemed to say "good luck with that" . I put my nose to it and it certainly smelled funky and in a state of deterioration (rot is probably a more appropriate description).What is it ?? Your guess is as good as mine.