Definition
To completely believe a deception or falsehood; to be easily tricked or fooled.
The scammer said he needed money to help a sick child, and she fell for it hook, line, and sinker.
He told me he was a billionaire, and I fell for it hook, line, and sinker, how foolish of me.
She swore she hadn't stolen my wallet, and I fell for it hook, line, and sinker.