Dick Turpin, How sharper than a serpent's tooth it is to have a thankless child!

Dick Turpin, Series 1, Episode 12. The Hostages

Sir John:  How sharper than a serpent's tooth it is to have a thankless child! William Shakespeare, Hamlet, you know.

Jem: King Lear!

Sir John: If I say it's Hamlet, then it's Hamlet! You presumptuous young puppy!

(serpent = a snake, especially a large one - literary)
(hostage = someone who is kept as a prisoner by the enemy so that the other side will do what the enemy demands)

You can learn more about this saying here The Phrase Finder
And of course it's King Lear, Sir John!