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

sig block
 /sig blok/, n.

    [Internet and Usenet; often written ?.sig? there] Short for ?signature?,
    used specifically to refer to the electronic signature block that most Unix
    mail- and news-posting software will automagically append to outgoing
    mail and news. The composition of one's sig can be quite an art form,
    including an ASCII logo, one's choice of witty sayings (see sig quote, 
    fool file), or even source code for small programs about which the author
    wishes to make a statement; but many consider large sigs a waste of 
    bandwidth, and it has been observed that the size of one's sig block is
    usually inversely proportional to one's longevity and level of prestige on
    the net. See also doubled sig, McQuary limit.


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

sig block

   /sig blok/ [Unix; often written ".sig" there].  See
   signature.

   [Jargon File]