When someone explains something or relates a story that starts in the story-teller's head and only half of the story is conveyed verbally; usually the second half. As if the story was Hampered by their own internal confusion and Half of their message actually reaches their vocal chords and is thus transferred past their lips.
Bill: "...so you should give Jim a call."
Greg: "Jim who?"
Bill: "What didn't you understand?"
Greg: "Everything that preceded SO! You pulled another Hamphalf! You should really consider a CAT Scan!"