Literally: "by the way" - an abbreviated version using the first letter of the first two words and substituting "dub" in play of the double-u sound
Person 1: (walks into house)
Person 2: "Hey, man. What's up?"
Person 1: "Nothing, just gotta take a slam."
Person 2: "Oh, okay. Hey, your girlfriend called a minute ago, b t dub."
A slangified vocalization of the popular Internet-communication abbreviation "btw," which stands for "by the way." The "dubs" is a shortened "double-u," which takes too long to say.
Another way of saying by the way which is in reference to the internet anagram btw (w, double-u, dub-el-u, dub, dub-zinski). also can be shortened to b.t. dubz.