Genesis 49:17

A chosen man shall arise from the house of Dan, like the basilisk which lieth at the dividing of the way, and the serpent's head which lurketh by the way, that biteth the horse in his heel, and the master from his terror is thrown backward. Even thus will Shimshon bar Manovach slay all the heroes of Philistia, the horsemen and the foot; he will hamstring their horses and hurl their riders backwards.

SH’PHIPHON (SERPENT). is the name of a snake which bites at the heel, with the second root-letter. being doubled. is shapha which means “hissing while biting,” except that the letter pei is here doubled. In the Jerusalem Talmud, Tractate Terumoth the Rabbis said, “It is a small kind of serpent, its name is sh’phiphon, and it is as thin as a hair,” with the earth being permitted to crack before it. Samson is compared to a snake because Samson did not wage war against his enemies as did the other judges and kings. Rather he alone went out against them, just as the viper which leaves his hole to attack travellers on the road, or the small species of serpent of which the traveller is not at all aware.

THAT BITETH THE HORSE’s HEEL.. This alludes to the two pillars upon which the house of Dagan, the Philistine god, rested, whereas the three thousand people who were upon its roof, Verse 27. are “the rider that falleth backward,” for when the snake bites the horse’s heels, the horse lifts its head and his forefeet, causing the rider to fall backward.

יהי, may the domain of Dan act like a snake barring the way, i.e. may he be the one killing nations hostile to the Jewish people

ויפול רוכבו אחור, may his enemies fall before him (off their horses.)

יהי דן נחש, a reference to a poisonous snake, one commonly called tzifoni, in Biblical literature. Arab physicians usually call such venomous snakes chorman. The name for such snakes implies that a single snake can kill numerous people at once, using both its stare, and it voice (hiss) which is heard and seen by many people. It is a fitting comparison to Shimshon who, all on his own, killed many people almost simultaneously.

שפיפון, name of a slim snake; our sages use this when wanting to illustrate something extremely thin, “as the width of a hair.” Physicians claim that these types of snakes hide in the trees, their sting being felt as if one had been shot by an arrow. Shimshon’s mode of operation, an individual lying in wait, in ambush, is compared by Yaakov to this snake.

יהי דן, Yaakov compared Dan to different kinds of serpents which do not travel (hunt) in packs, but rely on their wits, killing many. Shimshon, the most prominent warrior of the tribe of Dan, was a loner, never involving others when he faced danger. Compare Judges 15,8 ויך אותם שוק על ירך וכלחי חמור הכה אלף איש, “he struck them leg as well as thigh, and with the jawbone of a donkey he killed one thousand men.” He saved the Israelites from the constant incursions of the Philistines before he engaged in his acts of daring. This is what prompted Yaakov to wish him success with the words: 'לישועתך קויתי ה, “your type of salvation for Israel, orchestrated by a single individual, is something that normally, only G’d Himself can orchestrate.” Some commentators (Rashi) view these words of Yaakov as his reaction when he foresaw with prophetic vision how Shimshon had his eyes blinded by the Philistines. The words לישועתך וגו' then are Yaakov’s prayer that G’d Himself would avenge what was done to Shimshon. This prophecy/prayer was also fulfilled, as we are told in Judges 16,30 that Shimshon killed more Philistines in dying than he had killed cumulatively while alive.