Originally, it was a somewhat common mispelling of the word "brother", often by baby boomers, people attempting to imitate Hulk Hogan, and Fudds. It's now used often to trollpeople, and is used as a mild joke when shitting on boomers in gun communities, and biker communities. It's also especially popular on /k/, other boards of 4chan, when adding complete silliness to a post.
Boomer: do yuo like the 1911??/? IT won 2 worl wards shooting God's own caliber!
Normal Person: It's a decentweapon.
Boomer: AW HE'LL YEAH BORTHER!
Heck yeah borther: a phrase that started on YouTube, under the gaming channels Kryoz and SMii7Y. It’s more common on Kryoz’ channel, but is still often prevalent on SMii7Y’s, as well. It’s spoken in a southern accent, a simple mispronunciation of “heck yeah brother.” This phrase is a definite fan favorite.
“I just won the game.”
“You did?”
“Yeah!”
“Heck yeah!”
“Heck yeah, borther!”