Think of wicking like in terms of candles - the flame moves down the candle wick In terms of fabric/nappies, wicking is when wetness moves from the inside of the nappy, to the outside, either because of compression, or the outer fabric being tucked under, etc etc. It tends to be in cotton-outer nappies, things like Issy Bear Day Babies. Basically, somehow the moisture gets past the waterproof layer, and it ends up spreading across the outer fabric. Sometimes it can happen with vests and trousers too, wetness can wick onto them as well.