Noun, origin unknown.
One who offers up 'expert assessments' for the media in exchange for money.
Just about every field of human endeavor has its share of rent-a-quotes, who are always ready with a soundbite-sized opinion on some current issue or other in exchange for hard cash. Although sometimes well-informed and truly expert in their fields, they are more often individuals whose professional qualifications are far outweighed by their total confidence in their own opinions and their willingness to enter into pure conjecture at short notice on subjects about which they know very little.
An simple internet search for "rent-a-quote" turned up literally hundreds of examples from all over the world, which surprised me, since I thought it might be a purely British expression. But no: I immediately came across examples from the UK, the US, and Australia, as well as from the English-language media in India and Japan.
The following is a good example of the usage of the expression :
"Professor Steve Jones, of my alma mater, UCL, has one of the science of genetics' highest profiles, not to mention being one of the darlings of the British media and rent-a-quote whenever the Today programme need a science opinion."
(http://www.lowfield.co.uk/books/science.html)
This phrase seems to have replaced its 1970's equivalent, the vaguely offensive "rent-a-mouth", which I have to admit I preferred.