why is "in" used at the end of the sentence?

Native speaker’s answer
Rebecca
"In" is used at the end, as it forms part of the phrase "blow [something] in," which comes from the phrase "do [something] in." To "do [something] in" means to defeat or destroy it. It also indicates the movement of the destruction of the house into itself. Ex: They're gonna do the building in this week. = They're gonna demolish the building this week. Ex: My university work is going to do me in. Especially this semester.