I see this term getting thrown around a lot in the Hearthstone community and not a lot of context is there to fully understand its' meaning.
What does it mean if someone is "topdecking"?
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Top decking can be one of two things.
Your hand is empty and your hand each turns consists of the card from the top of your deck.
ex. "His hand is toast, he'll be top-decking for the rest of the game."
On your beginning-of-turn draw, you've drawn exactly the card you need in that particular situation in order to save yourself. (e.g. Flamestrike vs a hunter with a full board)
ex. "I can't believe he top-decked a Flamestrike! I had him next turn!"
As gatherer818 mentioned, this is a very old term originating in tabletop collectible card games. Here is a discussion thread on the Hearthstone forums about it.