how did grapefruit get its name--?

Answer 1

The grapefruit was known as the shaddock until the 1800s. Its current name alludes to clusters of the fruit on the tree, which often appear similar to grapes. Botanically, it was not distinguished from the pomelo until the 1830s, when it was given the name Citrus paradisi. Its true origins were not determined until the 1950s. This led to the official name being altered to Citrus × paradisi.

Answer 2

I think this might help:

"Grapefruit first recorded 1693 in Hans Sloane's catalogue of Jamaican plants; presumably it originated there from chance hybrids between other cultivated citrus. So called because it grows in clusters."