Bock is a style of lager beer that originated in Germany. It was traditionally brewed in the fall, at the end of the growing season, when barley and hops were at their peak. It was "lagered" all winter and enjoyed in the spring at the beginning of the new brewing season. Bocks can be pale (helles) or dark (dunkel) and there are double (doppel) bocks which are extra strong.

Bocks are usually strong beers made with lots of malt, yielding a very full-bodied, alcoholic beer. A persistent myth has been that bock beers are made from the dregs at the bottom of a barrel when they are cleaned in the spring. This probably seemed logical because of the heavier body and higher potency of bocks. From a brewing perspective, this is clearly impossible for two reasons - firstly the residue left after fermentation is unfermentable, which is exactly why it is left over; it cannot be fermented again to make more beer, and secondly, any attempt to re-use the dregs would probably result in serious bacterial contamination and a product that differs from beer as we know it.

Source : FAQ of alt.drinks.beer