Fat cats are great fodder for newspaper cartoonists. They are are usually smoking big fat cigars and their greedy grin screams "we got the cream". A giant Fat Cat flap is also occasionally drawn.
The word was first used in the 1920s in the US to describe rich political donors, but now it tends to be shorthand for those who are seen to have it easy at the expense of others.
In 2009, US President Barack Obama criticised "fat cat" bankers who pay themselves large bonuses.
There has been a gradual increase in the use of this term since the 1960s, says Brookes. From 2009-11, "fat" is the most commonly used adjective in front of "cat" - "pet" is second, followed by "stray", "pussy" and "scaredy".
The term was once aimed near-exclusively at people in the private sector, but now it's frequently used to describe those in the public sector.
Headlines such as "Town hall fat cats should be ashamed" and "The council 'fat cat' earning £570,000" are now typical.