“We’ve howdied, but we ain’t shook” means that we know each other, but we haven’t been formally introduced. The saying has also been used in Tennessee and is from the early 1900s.
 
 
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Tall Talk from Texas
by Boyce House
San Antonio: Naylor Company
1944
Pg. 96:
A West Texan was asked if he had met the new lawyer in the town. He replied, “We’ve howdied, but we haven’t shook.” 
 
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You All Spoken Here:
Southern Talk at Its Down-Home Best
by Roy Wilder, Jr.
Athens, GA: University of Georgia Press
1984 (1998)
Pg. 103:
Howdied but ain’t shook: Acquainted but never introduced.
 
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Body Language and Soulful Thoughts
by Petey Parker
Denton Texas: Zone Press, Rogers Publishing and Consulting, Inc.
2004
Pg. 67:
For anyone not born in the Lone Star State, the Texan accent and cowboy colloquialisms can seem a bi strange. Here is a guide to a few of the more colorful expressions you may encounter:
(...)
We’ve howdied but we ain’t shook yet=We’ve made a brief acquaintance, but not been formally introduced.
   
11 May 1953, Kingsport (TN) News, pg. 9 ad:
We’ve Howdy’d But Ain’t Shook! So We’d Like To Introduce
The Folks At Rook Realty Co.