Nintendinitis is a swelling of the thumbs caused by prolonged video game playing. The repetitive strain caused by holding onto the D-pad, pressing in the buttons, etc. can cause the pads of your thumbs to become leathery and feel sort of like they're "pressed in" against the nail (hard to explain, but it isn't pleasant). Usually the discomfort disappears within 24 hours, but in some extreme cases it can cause a permanent condition that affects the tendons in the hands, similar to tennis elbow.
Back when I used to play NES with my friends as a young child, we called this "Nintendo thumb". A more modern name for this type of injury is Blackberry thumb, which is for people who can't stop texting everyone.
Everything in moderation.