The Rumbar Pirates are a long dead pirate crew from the West Blue. They were first mentioned by Crocus as the group of pirates who brought Laboon to the Twin Capes.
The crew’s strength took them far into the Grand Line, however they met their end upon entering into the Florian Triangle. It is unknown who killed them, only that they died fighting an enemy they could not win against. As pirates, they were all prepared to die anytime, and as such, did not fear death. They only regretted that they must die before reuniting with Laboon.
Also it seems that many of them are very good at playing musical instruments. Brook even claims he can play all kinds of instruments, showing the ability to play at least violin, piano and guitar.
Furthermore, their sole surviving member (of sorts), Brook, was skilled enough to terrorize Thriller Bark along with its many undead inhabitants all on his own.
The name of the crew appears to be a combination of two similar words in a pun. The first term, “Rumba”, refers to a family of music rhythms and dance styles that originated in Africa. The second term, “Lumbar”, is a section of the body around the abdomen, known for the bones and spinal cord that exist there as part of the skeletal system. Due to the love of the music by the crew as well as due to Brook himself (whose skeleton, including his lumbar bones, are exposed due to the affect of the Yomi Yomi no Mi), it is presumed the two words were combined to define the name of the crew.
Despite being wiped out for about half-a-century, the Marines kept track of the crew’s information, as they managed to trace Brook’s connections based on the past bounty poster.