The sword of him that layeth at him - The word "sword" here (חרב chereb) means undoubtedly "harpoon," or a sharp instrument by which an attempt is made to pierce the skin of the monster.
Cannot hold - That is, in the hard skin. It does not penetrate it.
The spear, the dart - These were doubtless often used in the attempt to take the animal. The meaning is, that "they" would not hold or stick to the animal. They flew off when hurled at him.
Nor the habergeon - Margin, "breastplate." Noyes, "javelin." Prof. Lee, "lance." Vulgate, "thorax, breastplate." So the Septuagint, θώρακα thōraka. The word used here (שׁריה shiryâh), the same as שׁריון shiryôn 1-Samuel 17:5, 1-Samuel 17:38; Nehemiah 4:16; 2-Chronicles 26:14, means properly a "coat of mail," and is so called from its shining - from שׁרה shârâh, "to shine." It is not used in the sense of spear or javelin elsewhere, though perhaps it may have that meaning here - denoting a "bright" or "shining" weapon. This agrees best with the connection.

Habergeon - The hauberk, the Norman armor for the head, neck, and breast, formed of rings. See on Nehemiah 4:16 (note).

The sword of him that layeth at him cannot hold,.... It is either broken by striking at him, or however cannot pierce him and stick in him; but since a sword is not used in fishery, rather the harpagon or harpoon may be meant, which cannot enter into the crocodile, being so fenced with scales; but the whale being struck with it, it enters deep into his flesh, and is wounded by it; wherefore this and what follows in the next verses seems best to agree with the crocodile, or some other fish;
the spear, the dart, nor the habergeon; that is, neither of these can fasten upon him or enter into him: and yet it is certain that the whale, after he has been struck and wounded by the harping-iron, men approach nearer to him and thrust a long steeled lance or spear under his gills into his breast, and through the intestines, which dispatches him: darts are not made use of in the whale fishery; and as for crocodiles, as Peter Martyr says (c), they are not to be pierced with darts: the habergeon, or coat of mail, being a defensive piece of armour, seems not to be designed, as being never used in taking such creatures; rather therefore a javelin or hand dart may be intended; since, as Bochart observes, in the Arabic language such an one is expressed by this word.
(c) Apud Bochart. Hierozoic. par. 2. l. 5. c. 17. col. 785.

cannot hold--on his hard skin.
habergeon--coat of mail; avail must be taken by zeugma out of "hold," as the verb in the second clause: "hold" cannot apply to the "coat of mail."

26 If one reacheth him with the sword-it doth not hold;
Neither spear, nor dart, nor harpoon.
27 He esteemeth iron as straw,
Brass as rotten wood.
28 The son of the bow doth not cause him to flee,
Sling stones are turned to stubble with him.
29 Clubs are counted as stubble,
And he laugheth at the shaking of the spear.
משּׂיגהוּ, which stands first as nom. abs., "one reaching him," is equivalent to, if one or whoever reaches him, Ew. 357, c, to which בּלי תקוּם, it does not hold fast (בּלי with v. fin., as Hosea 8:7; Hosea 9:16, Chethb), is the conclusion. חרב is instrumental, as Psalm 17:13. מסּע, from נסע, Arab. nz‛, to move on, hasten on, signifies a missile, as Arab. minz‛a, an arrow, manz‛a, a sling. The Targ. supports this latter signification here (funda quae projicit lapidem); but since קלא, the handling, is mentioned separately, the word appears to men missiles in general, or the catapult. In this combination of weapons of attack it is very questionable whether שׁריה is a cognate form of שׁריון (שׁרין), a coat of mail; probably it is equivalent to Arab. sirwe (surwe), an arrow with a long broad edge (comp. serı̂je, a short, round, as it seems, pear-shaped arrow-head), therefore either a harpoon or a peculiarly formed dart.
(Note: On the various kinds of Egyptian arrows, vid., Klemm. Culturgeschichte, v. 371f.)
"The son of the bow" (and of the אשׁפּה, pharetra) is the arrow. That the ἁπ. γεγρ. תותח signifies a club (war-club), is supported by the Arab. watacha, to beat. כּידון, in distinction from חנית (a long lance), is a short spear, or rather, since רעשׁ implies a whistling motion, a javelin. Iron the crocodile esteems as תּבן, tibn, chopped straw; sling stones are turned with him into קשׁ. Such is the name here at least, not for stumps of cut stubble that remain standing, but the straw itself, threshed and easily driven before the wind (Job 13:25), which is cut up for provender (Exodus 5:12), generally dried (and for that reason light) stalks (e.g., of grass), or even any remains of plants (e.g., splinters of wood).
(Note: The Egyptio-Arabic usage has here more faithfully preserved the ancient signification of the word (vid., Fleischer, Glossae, p. 37) than the Syro-Arabic; for in Syria cut but still unthreshed corn, whether lying in swaths out in the field and weighted with stones to protect it against the whirlwinds that are frequent about noon, or corn already brought to the threshing-floors but not yet threshed, is called qashsh. - Wetzst.)
The plur. נחשׁבוּ, Job 41:29, does not seem to be occasioned by תותח being conceived collectively, but by the fact that, instead of saying תותח וכידון, the poet has formed וכידון into a separate clause. Parchon's (and Kimchi's) reading תוחח is founded upon an error.

Hold - Hebrews. cannot stand, cannot endure the stroke, but will be broken by it. The crocodile's skin, no sword, nor dart, nor musquet bullet can pierce.