The Collaborative International Dictionary of English v.0.48:

Imposthume \Im*post"hume\, n. [A corruption of aposteme. See
   Aposteme.]
   A collection of pus or purulent matter in any part of an
   animal body; an abscess.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Imposthume \Im*post"hume\, v. t. & i.
   Same as Imposthumate.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Aposteme \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.]
   [1913 Webster]