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1.
A generic phrase used in response to a question. The intonations of which this phase is said will assert the feelings of the person replying. If said happily, then the person is having a good day. If said sadly, then the person is having a day derived from the first word of the phrase. May also be used as a greeting or an explicative.
Origins believed to have come from in or near the vicinity of San Antonio, Texas.
As a response;
Bitsy: How y’all doin’ today, Clem?
Clem: SHIT HOWDY!!!

Bitsy: How y’all doin’ today, Clem?
Clem: …shit howdy.

As a greeting; Shit Howdy, ain’t it a glorious day!

As a explicative; Shit Howdy, I just won the lottery!
by Lee Pee January 27, 2004
 
2.
A greeting, primarily used in western states, expressing shock and amazement in association with the unexpected arrival of a former lover or spouse.
"Is that----? Shithowdy"
(Oh shit)
(Shit)
(Holy shit)
(No shit)
(Damn)
by hay-anus April 16, 2009