Turns out it's a pluot, an interspecific hybrid fruit with 3/4 plum and 1/4 apricot, and there are several varieties of pluots to boot (13 according to wikipedia). How is that possible. Well they are derived from a 1:1 hybrid of plum and apricot, the plumcot, which was generated simply by cross-pollination. The plumcot can then be crossed back to either an apricot or a plum tree to get different percentage contribution (which you could probably track by QTL analysis).
In fact you can do this with apricot, plum, cherry, peach and almond trees. For example the peacotum is a peach/apricot/plum hybrid which tastes just like fruit punch. Zaiger's Genetic from California holds the patent and trademarks to pretty much all these hybrids.
People sometimes mistake nectarines as a cross between plum and peach, but in fact nectarines are just a cultivar of peaches with a recessive mutation. The fuzziness of peaches is a dominant trait.
So there you go, we are still inventing new fruits, and there is still room for innovation by using simple crosses. Who knew those fruit trees were related enough to be crossed, and that the hybrids would be fertile. Now if one of them mutha uckas fruit vendors could make me a grapple, we would be in business...