Jamieson, Fausset & Brown

Say we not well that thou art a Samaritan, and hast a devil?--What intense and virulent scorn! (See Heb 12:3). The "say we not well" refers to Joh 7:20. "A Samaritan" means more than "no Israelite at all"; it means one who pretended, but had no manner of claim to the title--retorting perhaps, this denial of their true descent from Abraham.