The full form of HAMB is Hokey Ass Message Boards
The acronym HAMB represents the title of a forum for hot rod automobiles known as the “Hokey Ass Message Boards.”

Origin of HAMB
The HAMB has been around since 2003 and goes hand-in-hand with a website called The Jalopy Journal. The journal has been in existence since 1993 and the HAMB was launched circa 2003. Both are sites for dedicated owners of hot rod vehicles. The HAMB site is a message board where owners of these hot rod vehicles can share stories, show pictures, sell/buy parts, and sell vehicles.

Other Meanings
There is some evidence on the internet that in recent years the term hamb has come to mean cat also.

Example Conversations
A text conversation between two friends.

Friend 1: What are you up to today?
Friend 2: Not too much. I was going to work a bit on the hot rod but I can’t seem to find a part that I need for it.
Friend 1: Have you checked the HAMB?
Friend 2: Yeah, I put a message up yesterday but no one has responded yet.