According to their profile, the author of the post is a native German speaker. In German, the same word ("schwer") means both "heavy" and "difficult" or "seriou

According to their profile, the author of the post is a native German speaker. In German, the same word ("schwer") means both "heavy" and "difficult" or "serious" (in the sense that somebody in such a situation is carrying a heavy burden). The literal translation would be "heavy". English is a Germanic language, so perhaps in a sense this meaning has always been part of it. As you point out, even if it's not considered completely idiomatic most people would have no trouble interpreting the meaning.