The crackpot index is a number that rates scientific claims or the individuals that make them, in conjunction with a method for computing that number. The method, proposed (most likely as a joke) by mathematical physicist John Baez in 1998, computes an index by responses to a list of 34 questions, each positive response contributing a point value ranging from 1 to 50. The computation is initialized with a value of -5.
Presumably any positive value of the index indicates crankiness.
Though the index was not proposed as a serious method, it nevertheless has often been cited in discussions of whether a claim or an individual is cranky, particularly in physics.