V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

CSLIP
       Compressed [headers] Serial Line Internet Protocol (SLIP, IP)


The Free On-line Dictionary of Computing (30 December 2018):

Compressed SLIP
CSLIP

    (CSLIP) VanJacobsen TCP header compression.  A
   version of SLIP using compression.  CSLIP has no effect on
   the data portion of the packet and has nothing to do with
   compression by modem.  It does reduce the TCP header from
   40 bytes to 7 bytes, a noticeable difference when doing
   telnet with lots of little packets.  CSLIP has no effect on
   UDP, only TCP.

   (1995-05-28)