Here is an excerpt from Jules Verne's *De la Terre à la Lune*:

> 
> [...] les journaux de l'Union célébraient leurs inventions avec enthousiasme, et il n'était si mince
>  marchand, si naïf «booby» [Badaud.], qui ne se cassât jour et nuit la tête à calculer des trajectoires insensées.
> 
> 
> 

My understanding is this sentences tries to say that even the "mince marchand" and "naïf «booby»" are trying to calculate trajectories, but what is the grammatical function of the word "si" in this case?