A person with poor thumb dexterity. Can be used as either a noun or an adjective. Portmanteau of thumb and unco, the latter a popular Australian abbreviation of 'uncoordinated', meaning clumsy or possessing poor manual dexterity. Many people from Generation X and before are thuncos, not having grown up with text messaging, video games and other agents of repetitive strain injury.
I'll be a while typing this text message, I'm a total thunco.