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.
bf: "Honey i'm playing red dead redemption 2, i wish it would be more realistic..."
gf: "wanna be like a horse in rdr?"
bf: "lets do the howdy partner!"
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"