Mackeson, when used in the context of (yum) beer, usually refers to a particular stout.
The Hythe Brewery, in Hythe, Kent, was set up in 1699. William and Henry Mackeson acquired the brewery in the year 1801, and set about expanding production to cope with the demand for beer caused by the stationing of soldiers in the region in case of a French invasion. It continued operating for more than a century.
In 1907, various references say, the brewery first produced what is now called Mackeson Stout. It is noteworthy for being a milk stout - that is, a stout beer which has had lactose, or milk sugar, added during brewing. Lactose changes the character of the beer, because brewer's yeast cannot metabolize it to change into alcohol and/or carbon dioxide. As a result, the beer at finish is sweet and creamy - this, plus the additive, leads to the name 'milk stout.' As an aside, this means that lactose intolerant folks should be wary of drinking this beer. On the other side of the coin, wertperch informs us, this beverage was actually prescribed in hospitals for pregnant women!
In the late 1920s, Whitbread acquired control of the Hythe Brewery, and continued brewing the Mackeson Stout there until the brewery was closed in 1968, at which time the Stout brewing was moved to other Whitbread breweries. In 2000, Whitbread was acquired by the conglomerate InBev (formerly Interbrew), but fortunately production continues!
Presently, the Mackeson Stout is brewed in three regional varieties. The home market version is brewed in London at the Ram Brewery (by Young's) in London. It is three percent alcohol by volume and is available in cans or bottles. The United States has a version which is five percent ABV, and is brewed in Cincinnati, OH. That version is named Mackeson XXX, or "Mackeson Triple Stout." Finally, the Carib Brewery of Trinidad brews a 4.9% ABV XXX version for the Caribbean.
Try some. It's delicious.