Something fraudulent is intentionally false and meant to harm or deceive. That email message from the Sultan of Brunei offering you millions of dollars just might be fraudulent.
From the fact that the word fraudulent is frequently teamed with the word scheme, you can get the idea that something fraudulent is sneaky, snarky, and just plain wrong. A fraudulent scheme is one designed to gain something at the expense of someone else. It might be small — trying to use an expired coupon — or bigger — lying on your taxes. Whatever its form, planning something fraudulent is cheating, and it's wrong.