Jamieson, Fausset and Brown on Genesis 37:19

19. Behold, this dreamer cometh--literally, "master of dreams"--a bitterly ironical sneer. Dreams being considered suggestions from above, to make false pretensions to having received one was detested as a species of blasphemy, and in this light Joseph was regarded by his brethren as an artful pretender. They already began to form a plot for Joseph's assassination, from which he was rescued only by the address of Reuben, who suggested that he should rather be cast into one of the wells, which are, and probably were, completely dried up in summer.