A word commonly used in place of the word "fathom" by people who either don't know the difference or are simply ignorant. It is not the correct use of the word, as a phantom is a ghost and a fathom is a measure of nautical miles (so it is used as a verb when you cannot comprehend or "get to the bottom of" something); "phantom" is a noun, not a verb.
Dude1: Did you hear? Lindsay totally stood Quentin up yesterday!
Dude2: Really? Aw, I just can't phantom how someone could do that!
Dude1: Uh.... you mean fathom?