Several people have suggested thumb position, but it does not seem likely given it doesn't reach high enough, in my opinion.
Possibly it could mean left hand pizzicato, where the string is tapped energetically in the position of the note indicated, creating a very soft sound of that pitch. I have seen that marked with Xs in place of the note head.
I have also seen numerous notations to indicate bowing "on the bridge", which makes a very rough sound, or even under the bridge. Other possibilities include détaché or col legno (using the wood of the bow).
None of these techniques have universal notation, so they usually have a verbal indication above the score, and then adopt a symbol that was available to the typesetter indicating to which notes it applies. Possibly there is an earlier example in the score which has the extra verbal notation and it was omitted in this location?