A bitch in heat literally means a female dog who is in heat. (Heat meaning she is 'in season' to mate and have babies.)
A bitch in heat is sometimes used when reffering to humans, generally females. A female ththat goes after every male she can find, and is being extremely lustful. A blunt way to put this would be a horny woman.
1) A horny girl.
2) A dishonest/ lying person; comes from the old southern expression: "You lie like a dog(or bitch) in heat." (Femaledogs lie around a lot when in heat)
"Dude my piss stream just froze completely solid mid piss! It's colder than a bitch in heat!" and your friend says, "No shit it's colder than a bitch in heat This shit is hella Montanan yo!"