Dear Bingley: sorry I missed this when it was posted. I never heard the word.
A collection of pus or purulent matter in any part of an animal body; an abscess.

Origin: A corruption of aposteme. See Aposteme.

Source: Websters Dictionary
Webster's 1913 Dictionary

Definition: \Ap"os*teme\, n. [L. apostema, Gr. ? the separation of
corrupt matter into an ulcer, fr. ? to stand off: cf. F.
apost[`e]me. See {Apostasy}.] (Med.)
An abscess; a swelling filled with purulent matter. [Written
corruptly {imposthume}.]

I never heard this one before either.