why is a grapefruit called a grapefruit?

For those endearing people who want to do their best at making our whirlwind stopover in Canada enjoyable, i declare there should be GRAPEFRUIT JUICE at every dinner table. It was once available in the supermarket just downstairs from our flat, but for some reason or other, it's no longer stocked. A real shame. And right now, right this moment, i'm really really missing grapefruit juice.