Definition
Alternative form of thirteenth reason why.
How People Actually Use It
Bro, if someone said this to, you fucked up. As in reference to the Netflix Original 13 Reasons Why, Hannah Baker made 13 tapes of reasons on why she killed herself. If someone said “You’re my 13th reason why” You fucked up and the person is saying you are a reason they killed themselves.
"Person: I stuck my dick in cheese. Me:Wtf? You’re my 13th reason why."
Source: Urban Dictionary