IATA airline codes generally don't mean anything. Most older airlines (such as BA, AA) got "preferred" codes as those codes were available at the time they were assigned.

What did "DJ" have to do with "Virgin Blue"? Nothing.

The only aviation codes that actually mean something are ICAO assigned airport and aircraft codes.

The codes are just that. Something used to identify the "object" it has been assigned to. They don't mean anything.