I've always taken it to mean something to the effect that my allotted time has been used up (the candle is burnt out that we use for timing, maybe??), and the wick is getting low as well, so I'll be off. The snotter = the drip on the end of the nose, but probably here used metaphorically; the Scottish National Dictionary traces snotter, = "candle-snuff", to 1686. Cheers Murray