The charm by which the door of the robber's dungeon flew open. The reference is to the tale of The Forty Thieves, in the Arabian Nights.
“These words were the only `open sesame' to their feelings and
“The spell loses its power, and he who should hope to conjure with it would find himself as much mistaken as Cassim when he stood crying `Open, Wheat,' `Open, Barley,' to the door which obeyed no sound but `Open, Sesame.' ”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894