The Jargon File (version 4.4.7, 29 Dec 2003):
thanks in advance
[Usenet] Conventional net.politeness ending a posted request for
information or assistance. Sometimes written ?advTHANKSance? or
?aTdHvAaNnKcSe? or abbreviated ?TIA?. See net.-, netiquette.
The Free On-line Dictionary of Computing (30 December 2018):
thanks in advance
(Usually "TIA") A conventional Usenet
net.politeness ending a posted request for information or
assistance. Sometimes written "advTHANKSance" or
"aTdHvAaNnKcSe".
[Jargon File]
(1999-02-09)