In 353 AD, the Roman emperor Constantius II (337-361) withdrew from circulation all coins issued by his rival Magnentius in the western part of the Empire. The result was a drastic shortage of money in the West.
Local mints tried to fill this gap by issuing their own coins. They were modeled after imperial money, but since the skills of the local die-sinkers could not compete with those of the official Roman craftsmen, the imitations were mostly rather rough. This solidus is such a copy from a local mint and is thus called 'imitative.' In plain language, one would rather speak of a contemporary forgery, however.