The Cenimagni was a Celtic tribe, probably living somewhere in Southern England

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

 in the 1st century BC. They are known only from the writings of Julius Caesar

Julius Caesar

Gaius Julius Caesar was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire....

. It has been suggested that the name is a corruption of Iceni or Eceni Magni and actually refers to the better-known tribe of the Iceni

Iceni

The Iceni or Eceni were a British tribe who inhabited an area of East Anglia corresponding roughly to the modern-day county of Norfolk between the 1st century BC and the 1st century AD...

.

Following Caesar's military success and restoration of King Mandubracius

Mandubracius

Mandubracius or Mandubratius was a king of the Trinovantes of south-eastern Britain in the 1st century BC.-History:Mandubracius was the son of a Trinovantian king, named Imanuentius in some manuscripts of Julius Caesar's De Bello Gallico, who was overthrown and killed by the warlord Cassivellaunus...

 to power over the Trinovantes

Trinovantes

The Trinovantes or Trinobantes were one of the tribes of pre-Roman Britain. Their territory was on the north side of the Thames estuary in current Essex and Suffolk, and included lands now located in Greater London. They were bordered to the north by the Iceni, and to the west by the Catuvellauni...

, opposition to the Romans coalesced around Cassivellaunus

Cassivellaunus

Cassivellaunus was an historical British chieftain who led the defence against Julius Caesar's second expedition to Britain in 54 BC. The first British person whose name is recorded, Cassivellaunus led an alliance of tribes against Roman forces, but eventually surrendered after his location was...

 which led to divided loyalties among the Britons, as Caesar records. Emissaries of five British tribes, including the Cenimagni (the others being the Ancalites

Ancalites

The Ancalites were a Celtic tribe, probably living around the Thames Valley area in the 1st century BC. They are known only from the writings of Julius Caesar....

, the Segontiaci

Segontiaci

The Segontiaci were a tribe of Iron Age Britain encountered by Julius Caesar during his second expedition to Britain in 55 BC. They surrendered to him as he was campaigning against Cassivellaunus in the Thames Valley, which suggests they were also based in the south-east...

, the Bibroci

Bibroci

The Bibroci were a tribe of Iron Age Britain encountered by Julius Caesar during his second expedition to Britain in 55 BC. They surrendered to him as he was campaigning against Cassivellaunus in the Thames Valley, which suggests they were also based in the south-east....

 and the Cassi

Cassi

The Cassi are one of five tribes encountered by Julius Caesar during his second expedition to Britain in 55 BC when he crossed the Thames at Kew, and who became his allies after the Trinovantes joined him...

), arrived at the Roman camp to treat for peace, and agreed to reveal details of Cassivellaunus' stronghold. Caesar besieged him there and brought him to terms. When Caesar left Britain he took hostage

Hostage

A hostage is a person or entity which is held by a captor. The original definition meant that this was handed over by one of two belligerent parties to the other or seized as security for the carrying out of an agreement, or as a preventive measure against certain acts of war...

s from the Britons, although whether the Cenimagni were compelled to give any is not specified.