“We’ve howdied, but we ain’t shook”

“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.