The term “heart attack letter” does not have an established meaning in urban slang or the Urban Dictionary. However, if we break down the components, a “heart attack” can refer to a medical emergency involving the heart, while “letter” could imply some sort of written communication.
In some contexts, a “heart attack letter” might informally refer to a letter that induces a strong emotional or physical reaction, potentially due to shocking news, a breakup, or a significant life change that feels overwhelming. The idea would be that the letter is so intense that it metaphorically causes a “heart attack” due to the emotional weight it carries.
If you meant something specific or if there’s a particular context you’re referring to, please provide more details, and I’d be happy to help clarify!