Unburned and partially burned tobacco caked in the bowl of a pipe after smoking.
Etymology: early 1800s, diminutive of dot, lump.
The wikipedia article lists several potential causes of excessive dottle.
"Sherlock Holmes was, as I expected, lounging about his sittingroom in his dressing-gown, reading the agony column of The Times and smoking his before-breakfast pipe, which was composed of all the plugs and dottles left from his smokes of the day before, all carefully dried and collected on the corner of the mantelpiece." - Arthur Conan Doyle, "The Adventure of the Engineer's Thumb"