Ethernet: What is a jabber timeout
I did a quick glance at some other datasheets that reference a "jabber timeout".

From the MX98715AEC-C Ethernet chip:


  Transmit Jabber Timeout,
  indicating the MX98715 has been excessively active. The
  transmit process is aborted and placed in the stopped state. TDES0&lt;1> is also set


I would guess that the timeout setting stops the transmit process if the chip gets into some state where it wont "shut up" (ie it keeps jabbering on and on)

Hope that helps.