I'm not Armody, but I am from Texas: An "adder" (not real name) is a kind of small harmless snake, IIRC mostly black like a true cottonmouth moccasin without the white mouth. When agitated, he thinks he is a cobra, puffs up and acts mean.
Someone should have the real name. My book of Texas Snakes has no "adder" and I can't really find one that looks like the little fellow that used to be so plentiful out in the East Texas "Big Thicket" country where I grew up.