Why was Arthur John Priest known as "the unsinkable stoker?"

John Priest was one of the firemen, or stokers, of the 'Titanic' who had the task of keeping the boilers going day and night to power the electricity aboard the great ship. He was called "unsinkable" because he was one of the few who managed to escape the 'Titanic' alive when she struck an iceberg and sank in the icy Atlantic.

You can read more here:
https://www.bbc.com/news/uk-northern-ireland-17543632

He survived the Titanic and what other ship sinkings?

Arthur John Priest was a stoker, or fireman, on the RMS Titanic. His job was to shovel coal into the ship's boilers. He survived the Titanic's sinking of 1912. He also was aboard the RMS Olympic, the sistership to the Titanic, when she collided with HMS Hawke in 1911. Priest was also aboard the Britannic, the third sistership to the Olympic and the Titanic when she hit a mine and sank in 1916. Priest also survived the World War I losses of the Alcantara and Donegal. He once claimed that he was forced to retire from the sea by these disasters because no one wished to sail with him. Priest died at the age of 50 of pneumonia Southampton.

https://www.oldsaltblog.com/2016/04/titanics-unsinkable-survivors-arthur-john-priest-violet-jessop/