From The Free On-line Dictionary of Computing (8 July 2008) [foldoc]:
sig block
/sig blok/ [Unix; often written ".sig" there]. See
signature.
[Jargon File]
From Jargon File (4.4.4, 14 Aug 2003) [jargon]:
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.