Slang used in collectible card games, notably Magic: The Gathering. It is a short way to refer to drawing the card off the top of one's deck.
Usage:
Noun: "Staring down a horde of creatures, his topdeck would have to be amazing in order to surive."
Transitive verb: "He needed to topdeck Wrath of God to have any chance."
Intransitive verb: "His opponent had topdecked savagely over the previous few turns."
In general, this word has positive connotations. "Topdecking" is very close in meaning to "drawing the exact cards you need" if there's no other context. In general, good players are accused by bad players of better topdecking skills. As with most Magic slang, it is often used in a more ironic fashion, such as, "I topdecked 9 land in a row when a single burn spell would have won me the game."