What does "heavy" mean in air traffic control radio contact?

[Michael B]

I've listened to ATC radio conversations and sometimes hear the term "heavy" used. A generic example would be...

Houston Center to Northwest 1235 Heavy.
Northwest 1235 Heavy go ahead Houston Center.
Northwest 1235 Heavy descend and maintain 4000.
Descend and maintain 4000; copy, Northwest 1235 Heavy.