Europe in the middle ages was still a largely agrarian society, and the wealth of the nobility resided in their estates and land. Since most people still lived and worked on the land they would find farm implements instantly recognisable, (an important feature for a coat of arms), even if they seem obscure to us today, the tun is an example of this, a word long fallen out of common use but being a large barrel. Its use is probably in association with brewing or winemaking, we should not be surprised to find! A Glossary of Terms used in British Heraldry, J.H. Parker, Oxford, 1894, Entry:Tun