A Tweetup is a physical meeting of those whose usual association is via Twitter. A legal Tweetup is such an assembly intended (but not necessarily restricted) to those who have an interest in the law or in some aspect of legal practice, legal technology, legal education or justice.
They have been organised in London sporadically over the years, each one organised by a brave volunteer who manages the invitations and sets up a venue. A combination of living in the provinces and being often abroad means that I have attended only one of them.
The next one is organised by Louise Restell @Louise_Restell and fixed for Thursday 5 December at 6:30pm, probably in Chancery Lane. The invitation page is here. You will see that it is described as a Legal and associated hangers-on Tweetup.
I will not be there – I have a 24 hour trip to Washington which takes me away at just the wrong time.