adj: A determined, sneaky and fast opponent that is very hard to get rid of once they acquire their target.
That guy's a piranha if you have to fight him - he'll beat you down no matter how long it takes.
adj: Something that's more intense than it really "needs" to be. Often associated with personality or liquids. Such as someone being too funny for their own good - they just keep telling the same jokes until they aren't funny anymore. Or a mixed drink that has too much alcohol in it to taste good.
Likely origin: Taking too much Vitamin B (folic acid) makes your urine very dark and potent smelling - or "folic".
Those margaritas are so folic it's like doing a shot of cheap tequila.