Well, a carp is a fish, and meso- means middle, and so...
...but no, you are much too sophisticated to fall for that. A carp is a fish, certainly, but the ending -carp implies a fruit (yes, that's right, it's Greek).
But what exactly is a mesocarp? What can a middle fruit be?
Mesocarp sounds rather nasty, whatever it is.
Well, the mesocarp of an almond will have disappeared long before you buy it, and the mesocarp of a walnut will also have disappeared (unless you're eating it pickled); but the mesocarp of an orange or lemon will still be there. You'll probably call the pith.
And, yes, that is quite nasty.
But, on the other hand, when you think of the great dripping, slurping, sugary pleasure we get from the flesh of an apricot:
photo by xamumu
or the intense joy to be had from eating a ripe glistening mango, or a glowing fragrant peach, it seems a great crime that botanists have christened the flesh of those fruits the mesocarp, too.
illustration by LadyofHats
Word Not To Use Today: mesocarp. As you know, meso- is Greek for middle, and -carp is Greek for fruit.
As you'll also know, sinking your teeth gently into the melting flesh of a warm peach is much more fun than biting a chunk out of its mesocarp.