An established(ie: long used) term referring to WW2 vets, however, it was self given(I think..) and is generally used in one of two ways...
1)arrogantly, often something along the lines of the first definition for this word(^)
2)by pacifists who wish to not seem that way or who wish to not be thought of as ungrateful
"Hey! We're the Greatest Generation you hippie punks!"
"What sacrifices, they truely are the greatest generation..."