Behold, this dreamer cometh - בעל החלמות baal hachalomoth, this master of dreams, this master dreamer. A form of speech which conveys great contempt.

And they said one to another,.... According to the Targum of Jonathan, Simeon and Levi said what follows: nor is it unlikely, since they were hot, passionate, cruel, and bloody minded men, as appears by the affair of Shechem; and perhaps this may be the reason why Joseph afterwards, when governor of Egypt, took Simeon and bound him, Genesis 42:24; which was but a just retaliation for his advice to cast him into a pit when slain:
behold, this dreamer cometh; or "master of dreams" (r); not of the interpretation of them, but of dreaming them; that had them at his command when he pleased, as they jeeringly flouted him; as if he was a framer and contriver of them, and only pretended to them when he had none, or else that he was frequently dreaming and telling his dreams; this they said in a sarcastic way, and, perhaps, as pleased, and rejoicing that such an opportunity offered to take their revenge on him: this shows that it was on the account of his dreams chiefly that they bore such a grudge against him, that this was uppermost on their minds, and was revived at first sight of him, and from whence their malice sprung.
(r) "magister somniorum", Tigurine version, Montanus; "dominus somniorum", Vatablus, Junius & Tremellius.

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.