The Jargon File (version 4.4.7, 29 Dec 2003):

asbestos longjohns
 n.

    Notional garments donned by Usenet posters just before emitting a remark
    they expect will elicit flamage. This is the most common of the asbestos
     coinages. Also asbestos underwear, asbestos overcoat, etc.


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

asbestos longjohns

    Notional garments donned by Usenet posters just
   before emitting a remark they expect will elicit flamage.
   This is the most common of the asbestos coinages.  Also
   "asbestos underwear", "asbestos overcoat", etc.

   [Jargon File]

   (1997-07-04)