It can be compared to 'the yips', an expression used to describe apparent loss of fine motor skills without any explanation.
The term is used in reference to players who struggle with some kind of psychological problem affecting their ability to release their darts.
[1] It is described by the Collins English Dictionary as "(in darts) nervous twitching or tension that destroys the concentration and spoils performance".
[5] In the 2017 Grand Slam of Darts, Berry van Peer's struggle with dartitis was visible in his matches against Simon Whitlock, Gary Anderson and Cameron Menzies.
[8] Dartitis is believed to be a form of dystonia,[6] which is described by the UK NHS as: A medical term for a range of movement disorders that cause muscle spasms and contractions.