An adjective used to describe someone (very) active on Twitter.
For the average internet-goer, being Twitterish only lasts for a short period of time after a Twitter account is acquired.
Becoming Twitterish may occur after someone realizes that have a large sum of followers (more than one), and feel that they would be let down if that someone doesn't tweet.
Nancy: Dude, ever since Johnny realized he had 3 followers, he's been so Twitterish! He dumped me "for the good of the community!"