this is where i came in

this is where I came in

This is where I began, my knowledge dates from this point. For example, Do you have anything more to add, because if not, this is where I came in. This idiom, dating from the 1920s, originally alluded to the continuous showing of a motion picture, with customers entering the theater at any stage while the film was running and leaving when it reached the point where they had started.