As in reference to the British tank used in WWII, now denotes large females found at parties and other social gatherings (especially involving alcohol) that attempt to get play from unsuspecting males. Often, they are slow and clumsy, and are always extremely bulky. Approach with care.
In such conversations Sherman is often referred to as a person, such as saying "is Sherman around?" to ask a dealer if he/she currently has any shrooms available for purchase.