Where does the card game "Poker" get its name from?

Poker is a game that is German in origin, and gets its name from the German pochen, meaning "to boast or brag"- though the literal translation is "to knock." A "knock" is still used in poker to indicate a passed bet, and the game still includes considerable "bragging." However, the game appears to have been introduced to this country through New Orleans, where, due to its similarity to the French card game of poque, it acquired that name. Southern gentlemen who played the game but did not know French pronunciation tried to say "pok-uh", which is very similar to the pronunciation of "poker" today. Northerners that learned the game deduced, therefore, that it was spelled like it sounds.

Another good example of traditional American word-butchering is "Hocus Pocus".
Sources:
An insert card into a 52-card deck by Hoyle
Webster Dictionary research