Why is it called a grapefruit? There is no immediate resemblance between a grape and a grapefruit. The most common theory is that it in allusion to the way the fruit grows in clusters on the tree, resembling grapes. This is not what seems like the most likely theory to me. Another, less popular, but my favored theory is that the name comes from its scientific moniker. Citrus grandis translates to great citrus or great fruit. So perhaps, the grapefruit was called the greatfruit. This indeed would be a good name for this giant breakfast orange.
But we all know that language is formed between the constraints of the lazy tongue and the strained ears so perhaps we started calling it grapefruit for the of ease of the tongue and the misinterpretation of the ears.