Dan shall be a serpent by the way, an adder in the path, that biteth the horse heels, so that his rider shall fall backward.

King James Translators' Notes

an adder: Heb. an arrow-snake

Jamieson-Fausset-Brown Bible Commentary

17. Dan-"a judge."

a serpent . an adder-A serpent, an adder, implies subtlety and stratagem; such was pre-eminently the character of Samson, the most illustrious of its judges.

Matthew Henry's Concise Commentary

49:13-18 Concerning Zebulun: if prophecy says, Zebulun shall be a haven of ships, be sure Providence will so plant him. God appoints the bounds of our habitation. It is our wisdom and duty to accommodate ourselves to our lot, and to improve it; if Zebulun dwell at the heaven of the sea, let him be for a haven of ships. Concerning Issachar: he saw that the land was pleasant, yielding not only pleasant prospects, but pleasant fruits to recompense his toils. Let us, with an eye of faith, see the heavenly rest to be good, and that land of promise to be pleasant; this will make our present services easy. Dan should, by art, and policy, and surprise, gain advantages against his enemies, like a serpent biting the heel of the traveller. Jacob, almost spent, and ready to faint, relieves himself with those words, I have waited for thy salvation, O Lord! The salvation he waited for was Christ, the promised Seed; now that he was going to be gathered to his people, he breathes after Him to whom the gathering of the people shall be. He declared plainly that he sought heaven, the better country, Heb 11:13,14. Now he is going to enjoy the salvation, he comforts himself that he had waited for the salvation. Christ, as our way to heaven, is to be waited on; and heaven, as our rest in Christ, is to be waited for. It is the comfort of a dying saint thus to have waited for the salvation of the Lord; for then he shall have what he has been waiting for.