The Collaborative International Dictionary of English v.0.48:

Jam \Jam\, n. [Prob. fr. jam, v.; but cf. also Ar. jamad ice,
   jelly, j[=a]mid congealed, jamd congelation, ice.]
   A preserve of fruit boiled with sugar and water; also called
   jelly; as, raspberry jam; currant jam; grape jam.
   [1913 Webster]

   Jam nut. See Check nut, under Check.

   Jam weld (Forging), a butt weld. See under Butt.
      [1913 Webster]