I hear this term in reference to Warlocks. I think it's about how Warlock's keep a ton of burn spells in their hand and hit you directly. Not sure if this is accurate or not.
The Handlock is a Warlock deck that includes Twilight Drakes and Giants. They're called Handlocks because their strategy involves tapping on their early turns to get a big hand quickly to get big Twilight Drakes and cheap Mountain Giants.