An adjective used to describe someone (very) active on Twitter.
For the averageinternet-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!"
"Twittering" is a word mistakenly used to describe the action of posting a comment on Twitter. It is primarily said by people who have never used Twitter before, and only heard about it from and friend or a T.V show.
The proper word would be "tweeting" (see tweeting)
Hey man, did twitter that message to your friends? - Guy 1
No, but I tweeted the post to my followers... - Guy 2
I think I'm going to be twittering this link to everyone I know! - Guy 1