1
general::
A wealthy and privileged person, as in This neighborhood, with its million-dollar estates, is full of fat cats. This term originally meant "a rich contributor to a political campaign," and while this usage persists, it now is often applied more broadly, as in the example. [Colloquial; 1920s]
American Heritage Idioms
2
general::
Phrase(s): fat cat
Fig. someone who is ostentatiously and smugly wealthy. • I like to watch the fat cats go by in their BMWs. • I’m no fat cat. I can’t even pay my normal bills!
McGrawhill's American Idioms And Phrasal Verbs