A playful, emphatic way of saying “no excuses, no arguments, end of discussion.” It’s basically “no ifs, ands, or buts allowed”. What you mean is “final answer, no debate,” just with a rhyme so it lands soft instead of savage.
Basically to avoid sounding “rude”.
It’s a modern spin on the much older phrase “no ifs, ands, or buts,” which has been used for decades to mean “no exceptions.” The “coconuts” addition is a newer, internet-era twist, rhyme-driven wordplay that turned a stern phrase into something silly enough to say to a kid, a friend, or even yourself without sounding like a drill sergeant.