Either the head is so hydra,
The hydra is a creature in greek mythology that when you cut off one of its heads, two more would grow back in its place. He is saying that if you keep messing wit him he will keep coming at you even harder each time, (hydra grows 2 heads every time a head is cut off), or you can be cool and let bygones be bygones.
Also notice the play on head/hydra (wet) — sexual innuendo
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