1. An unneutered male ferret, of any age. (A neutered male ferret is a gib.)
2. In the UK, hob may be used to mean the stovetop or a hot plate. ‘Hob’ may refer specifically to a flat metal or ceramic disk on a stovetop that covers the heating element (or, in older wood-burning stoves, heats up in the absence of a specific heating element). This most likely comes from the definition Webster mentions; a small shelf in or near a fireplace that is used for heating food.
3. Hob is the name of the spike of marker that is used in the game of quoits.
4. To play hob is an English idiom meaning to mess up -- "Alas, the swirling dust devils played Hob with their cameras". It also means to act in a mischievous manner -- "Ed is playing hob". This is a reference to the small mischievous spirit Hob, whose name is probably an abbreviation of Robin Goodfellow (Hob was a Middle English abbreviation for Robin).
5. HOB is also short for High-Order Byte.