This makes reference to Adam and Eve being thrown out of the Garden of eden after Eve ate the forbidden apple - a cherubim with a flaming sword was put there "to guard the way to the tree of life". (If you have a bible handy you can read it at Genesis chapter 3 verse 24). So presumably the bared arm refers to the arm of the cherubim brandishing the flaming sword.

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"Significantly, Frankenstein compares himself and Elizabeth to Adam and Eve. He says that his "paradisiacal dreams of love and joy" are dashed by the realization that "the apple was already eaten, and the angel's arm bared to drive [him] from all hope." This Biblical allusion has a number of ramifications. The apple of which Eve ate came from the Tree of Knowledge, which God had forbidden them to touch; it was for their curiosity that the first people were cast out of Paradise. Similarly, Frankenstein's misfortune befell him as a result of his overweening scientific curiosity and his desire to defy the work of God".
http://www.gradesaver.com/frankenstein/study-guide/section7/