The definition of too good to be true is:
(idiom) too positive to be real or believable
A: I found an apartment on Craigslist that is 3 bedrooms and only $500!
B: That sounds too good to be true.
A: Why? The landlord just wants me to send a check to Nigeria to confirm the deal….
A: Her new boyfriend is a Fireman, has a lot of money and is so handsome!
B: He sounds too good to be true. Is there anything wrong with him?
A: He’s married….
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