As the name implies: A chemical produced by a fungus; from Rapa Nui, the Easter Island. English language article from Max Planck Institute.
And one at Daily Mail.
P.S.: Turns out Rapamycin or Sirolimus as it is also called is known since 1972 and used as an immunosuppressant drug in kidney transplants. So that’s a side effect you don’t necessarily want. Also, a 5 year trial with 1500 dogs of 25kg or above weight is underway to test whether it increases their life span. It worked in mice before with increases of 20 to 30% life span. The Dog study should be finished in 4 years or so.