Although it had been suggested by E. Dijkstra in a paper already decades ago, it remained unnoticed for a long time. Only recently it has been rediscovered and hyped to death with programming languages such as Ruby and Python. (Sometimes falsely referred to as 'duck typing').
Definition: "If it smells like a dick and tastes like a dick, it must be a dick".
Bob: Nah, the problem is, if it smells like a dick and tastes like a dick, it could be a dragon doing a dick impersonation. You may not always want to let dragons into your ass, even if they can impersonate a dick.