The traditional greeting of southern, conservative, gun-loving, poor, white farmers, who also think the Confederate flag is the flag of the United States.
The term is also associated with hoboes, hicks and rednecks.
Rarely is the person to whom the gretting is addressed any sort of 'partner'. Except, possibly, in the context of an incestuous relationship between the two parties.
Hick 1: Howdy partner! What y'all doin' tonight?
Hick 2: I'm gonna 'ave me some cows for dinner!
Hick 3: I'm gonna go shoot me some trespassers with ma 12-gauge! I'll bring back the meat!
Hick 1: K, howdy y'all, I best be off, ma Doris want's some hot lovin, ya know!
Hick 2: Doris yo sister or Doris yo mothar?
Howdy, partner! Ya'll from 'round these parts?