A Grindolf is someone that grins a lot (hence "grin") and likes to lick dolphins (hence "dolf"), this is a very rare insult to throw at someone; not many people do both of these simultaneously.
George: You are such a grindolf, man!
Gary: What?! Man, if anyone is a grindolf around here.. it's YOU! You're the one who grins and licks dolphins!
George: I know babe, I was just checking ya knew what it meant! And... YOU DO! Nice work, Gar ;)