A
dismissive and
derogatory term for an
outmoded or
irrelevant person or
institution.
Though of uncertain origin, this particular usage first became widespread in England in the popular music press during the heyday of Punk Rock, around 1976-77. The two main weekly popular music tabloids at the time, 'New Musical Express' (aka NME) and 'Melody Maker', denounced as a dinosaur practically every performer who was popular before 1976. The expression was quickly taken up by the media in general, and its meaning widened to include anyone or anything that was deemed to be out of date.
It is somewhat ironic that Mark Knopfler, who as the leader of stadium rock band 'Dire Straits' was among the many artist dismissed as a dinosaur, now has an actual dinosaur named after him (Masiakasaurus knopfleri). I suppose it would have been rather more ironic if John Lydon (aka Johnny Rotten) had one named after him though (Masiakasaurus rotteni?).