What is to come unexpectedly; -- with in or into; as, my old friend dropped in a moment. Steele. Takes care to drop in when he thinks you are just seated. Spectator called?

drop — To come unexpectedly; -- with in or into; as, my old friend dropped in a moment. Steele. Takes care to drop in when he thinks you are just seated. Spectator. (Webster's 1913.)

Sources