"How sharper than a serpent's tooth it is to have a thankless child!" - This quotation is from the play -

• "How sharper than a serpent's tooth it is to have a thankless child!" - This quotation is taken from William Shakespear'e play King Lear.

Some other important quotation from King Lear by William Shakespeare -

• Nothing will come of nothing: speak again.

• O! let me not be mad, not mad, sweet heaven;
Keep me in temper; I would not be mad!

• I am a man
More sinned against than sinning.

• O! that way madness lies; let me shun that.

• When we are born we cry that we are come
To this great stage of fools.

• Speak what we feel, not what we ought to say.

Source: Oxford Essential Quotation