It's
\Spong\, n. [Etymol. uncertain.] An irregular, narrow, projecting part of a field. [Prov. Eng.]
According to dictionary.com, at least. ISTR that the OED says it's something different but I can't be arsed to get out of my chair and go down to the Library.
(
foop,
Mon 2 Jun 2003, 13:37,
archived)