The Collaborative International Dictionary of English v.0.48:

Maypop \May"pop\, n. [Perh. corrupt. fr. maracock.] (Bot.)
   The edible fruit of a passion flower, especially that of the
   North American Passiflora incarnata, an oval yellowish
   berry as large as a small apple.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Passiflora incarnata
    n 1: of southern United States; having an insipid berry the size
         of a hen egg [syn: maypop, Passiflora incarnata]