THX 1138 is the license number of his old Ford Falcon that he wrecked, or somthing. I also heard somewhere he had a malamut dog that liked to chew on things about the same time... but it could be urban legend.
IMDB says the dog was real and is named Indiana. THX 138 was the license plate of a car in Graffiti (Ford Falcon?). In truth it was a way for him to remember his phone number. Well that clears that up...
