A little more seriously, I think an 'oil can resonance' occurs when the tweeter dome moves out of phase with the movement of its surround - in other words, the surround is moving the tweeter outward but the center of the tweeter dome is actually moving inward, and vice versa. This is all related to the frequency, the mass of the dome, inertia, and a bunch of other physics terms I can't remember.
I think - I'm guessing - that the terminology comes from the old style of oil cans somehow?