Mull can mean to think ponderously or deeply. E.g. Patsy mulled over an idea for her next novel. I do a lot of mulling.
Alternatively it's a soft muslin fabric, a layer of humus, or the practice of heating liquid, usually wine, with spices to make a delicious warming drink.
There's a good reason for Mull being today's word of the week – it's the Scottish island in the inner Hebrides where the mobile writing retreat is currently located.