your blood is far too thin

[thod00]

Its certainly not a common expression and I would require further context to give my interpretation. I would guess that it means she is saying that he is in some way unworthy to marry her. An alternative would be "you are not man enough".

I think that the 'blood is thicker than water' theme is misleading since that idiom has a very specific and unrelated meaning.

[Raymott]

Its certainly not a common expression and I would require further context to give my interpretation. I would guess that it means she is saying that he is in some way unworthy to marry her. An alternative would be "you are not man enough".

I think that the 'blood is thicker than water' theme is misleading since that idiom has a very specific and unrelated meaning.

I agree. It's completely off-topic. That happens sometimes, and even worse is when the tangential theme takes off even before the OP has an answer. However, in this case, the OP got several relevant answers first.

[Torrance88]

Well, blood thins from using alcohol...

...I don't have a clue what it means. We still don't have an answer as to in what degree he's unworthy of her hand in marriage.

[crossmr]

It is far more likely that she either means that the person is physically weak or that he is of an undesirable social group.

Thin blood was taken to indicate a low system - whereas good, thick red blood indicated good health.

I tend to agree with this analysis. Whenever I've heard this idiom I've heard it used in reference to people who are weak or cowardly, often both.