how sharper than a serpent's tooth it is to have a thankless child

The phrase "how sharper than a serpent's tooth it is to have a thankless child" is a quotation from William Shakespeare's play "King Lear." It represents the pain and bitterness experienced by a parent who has a child that lacks gratitude and appreciation. It suggests that such ingratitude is worse than being bitten by a snake, emphasizing the depth of disappointment and sorrow caused by a child's lack of gratitude.