So I think this is one of the downsides of learning a language through literature. This is a translation of Thomas Mann's "Der Zauberberg" with the following relevant passage : "Joachim Ziemßen nicht mal ganz fest auf der Brust..." I think the expression might not be so common, even in German (from a quick check on Google). I believe the translation into English was more or less faithful to the literal German, but the expression does seem odd in English. I would only use the the expression, "weak in the legs", but even this sounds somewhat archaic. I would certainly not imitate any expressions like "weak in the lungs" or "weak in the heart". If you are going to learn the colloquial language through literature, you should read the original text. I think much gets lost in translation.