Grapefruit
early 19th century: from grape + fruit (probably because the fruits grow in clusters).
wiktionary
Widely assumed to be a marketing term from grape + fruit, an allusion to the supposed grapelike clusters of fruit on the tree [1], early 19th c. Ciardi proposes another theory: one of the pummelo's botanical names is Citrus grandis, meaning "great citrus [fruit]", due to the size of its fruit. A new pummelo variety might first have been called a "greatfruit" (see greatfruit), and through the process of dissimilation, the word came to be pronounced "grapefruit".
etymonline
grapefruit (n.)
1814, from grape + fruit. Said to have been so called for its taste, or perhaps because it grows in clusters. Perhaps a marketing name; it was known by various names (pomelo, shaddock) before the current one emerged. The fruit itself was known since 1693 (in Hans Sloane's catalogue of Jamaican plants); presumably it originated in Jamaica from chance hybrids between other cultivated citrus. An ornamental plant chiefly at first, not much eaten until late 19c.
