A nickname for the Yeoman Warders of the Tower of London, its origins are somewhat obscure but apparently related to a penchant for beef and was presumably meant as an insult (hence they prefer being called by their proper title). It was apparently first used of the English population in general and is said to have first been applied to the Yeoman Warders in the second half of the 17th century.

The Yeoman Warders of the Tower of London (not to be confused with the Yeomen Warders of The Guard, the Sovereign’s Bodyguard, who are also referred to as “beefeaters”), are charged with guarding of the Tower of London and its contents including the prisoners of state who were formerly held within its walls (the last was during World War II).
With a history dating back to the reign of King Henry VII and first present at the Tower during the reign of King Henry VIII, they were made extraordinary members of the Sovereign’s Bodyguard during the reign of King Edward VI in 1552 (meaning they wear scarlet livery and carry partizans on state occasions).
The Chief Yeoman Warder is their commander and second-in-command is a Yeomen Gaoler (who still carries an axe on state occasions). There are also three Sergeant Yeoman and a Yeoman Ravenmaster. A Yeoman Warder (along with a detachment of soldiers) carries out the Ceremony of the Keys every night – formally locking the Tower.
These days (drawing on innovations introduced by the Duke of Wellington when he was Constable of the Tower between 1826 and 1852), the Yeoman Warders must be warrant officers who have served at least 22 years (and have been awarded the long service and good conduct medal) in the British Army, Royal Air Force and Royal Marines.
The Yeoman Warders – and there were 38 (although we’re not sure how cutbacks due to COVID may affect this) – live within the Tower under the authority of the Resident Governor. They wear the red state dress uniform on state occasions and a dark blue “undress” uniform for everyday use. Moira Cameron became the first female Yeoman Warder in 2007.
As well as carrying out state duties, since Victorian times the Yeoman Warders have conducted towers of the Tower and assisted visitors with their inquiries.
Why do the Yeoman Warders have to be warrant officers who have served at least 22 years (and have been awarded the appropriate medal) in the British armed forces? Is it a financial reward for ex-servicemen who are now middle aged or older?