There may be a number of symbolic interpretations of this gesture. The previous answer discusses Arnold Friend as a possible representation of 'the devil." I personally don't think this is a literal intention of the story, but perhaps one that has some subtle symbolic intrigue to add. The 'X' friend draws in the air may have any number of prosaic meanings for him. For example he may be 'marking' Connie as his next conquest or property. The proverbial 'X' on treasure maps means he considers her a treasure, and given her beauty this interpretation makes sense. The 'X' may also signify Friend's intention to seduce Connie, making her another 'notch' in his belt. Finally, the symbol could also show that Friend intends to harm or kill Connie, eliminating her or 'crossing her out.' In any case, it's a very chilling moment both for the reader and for Connie. Interestingly, in the film version (Smooth Talk) the 'X' becomes a kind of curvy 'S' or figure 8 design.
Many believe this to be another example of Arnold Friend's symbolism as a Satan figure, or even as Satan himself. Making the mark of an X in the air could arguably be Satan's mark, supporting the theory that he is, at the very least, a symbol of Satan. Further evidence to prove this point exists in that he appears to have cloven hooves due to how he is described as standing off balance in his boots. Also, he has the ability to "see" where Connie's family is, and he seems to be unable to enter her house - it needs to be her choice to come to him, just as the devil is believed to need his followers to choose sin for themselves.
the Xin the air symbolizes a sideways cross, and arnold cannot enter the house because the devils folloers must go to him he cannot get them. the story endsd on sunday or "son-day" symbolzing the son of god , and arnold puts his glasses on his head stratigically to cover his "horns"