Solomon 8:6

Set me as a seal upon thine heart, as a seal upon thine arm: for love is strong as death; jealousy is cruel as the grave: the flashes thereof are flashes of fire, a very flame of the LORD.

Clarke's Commentary on Song of Solomon 8:6

Set me as a seal upon thine heart - It was customary in the Levant and other places to make impressions of various kinds upon the arms, the breast, and other parts. I have seen these often: some slight punctures are made, and the place rubbed over with a sort of blue powder that, getting between the cuticle and cutis, is never discharged; it continues in all its distinctness throughout life. The figures of young women are frequently thus impressed on the arms and on the breasts. If the bride alludes to any thing of this kind, which is very probable, the interpretation is easy. Let me be thus depicted upon thine arm, which being constantly before thy eyes, thou wilt never forget me; and let me be thus depicted upon thy breast, the emblem of the share I have in thy heart and affections. Do this as a proof of the love I bear to thee, which is such as nothing but death can destroy; and do it to prevent any jealousy I might feel, which is as cruel as the grave, and as deadly as fiery arrows or poisoned darts shot into the body.
A most vehement flame - שלהבתיה shalhebethyah, "the flame of God;" for the word is divided שלהבת יה shalhebeth Yah, "the flame of Jehovah," by one hundred and sixteen of Dr. Kennicott's MSS., and by one hundred and fourteen of those of De Rossi. It may mean the lightning; or, as our text understands it, a most vehement or intense fire.

Barnes' Notes on Song of Solomon 8:6

The bride says this as she clings to his arm and rests her head upon his bosom. Compare John 13:23; John 21:20. This brief dialogue corresponds to the longer one Cant. 4:7-5:1, on the day of their espousals. Allegorical interpreters find a fulfillment of this in the close of the present dispensation, the restoration of Israel to the land of promise, and the manifestation of Messiah to His ancient people there, or His Second Advent to the Church. The Targum makes Sol 8:6 a prayer of Israel restored to the holy land that they may never again be carried into captivity, and Sol 8:7 the Lord's answering assurance that Israel henceforth is safe. Compare Isaiah 65:24; Isaiah 62:3-4.