But the tongue can no man tame - This does not mean that it is never brought under control, but that it is impossible effectually and certainly to subdue it. It would be possible to subdue and domesticate any kind of beasts, but this could not be done with the tongue.
It is an unruly evil - An evil without restraint, to which no certain and effectual check can be applied. Of the truth of this no one can have any doubt, who looks at the condition of the world.
Full of deadly poison - That is, it acts on the happiness of man, and on the peace of society, as poison does on the human frame. The allusion here seems to be to the bite of a venomous reptile. Compare Psalm 140:3, "They have sharpened their tongues like serpent; adders" poison is under their lips." Romans 3:13, "with their tongues they have used deceit; the poison of asps is under their lips." Nothing would better describe the mischief that may be done by the tongue. There is no sting of a serpent that does so much evil in the world; there is no poison more deadly to the frame than the poison of the tongue is to the happiness of man. Who, for example, can stand before the power of the slanderer? What mischief can be done in society that can be compared with that which he may do?
- 'Tis slander;
Whose edge is sharper than the sword; whose tongue
Outvenoms all the worms of Nile; whose breath
Rides on the posting winds, and doth belie
All corners of the world: kings, queens, and states,
Maids, matrons, nay, the secrets of the grave
This viperous slander enters.
Shakespeare in Cymbellna.

But the tongue wan no man tame - No cunning, persuasion, or influence has ever been able to silence it. Nothing but the grace of God, excision, or death, can bring it under subjection.
It is an unruly evil - Ακατασχετον κακον· An evil that cannot be restrained; it cannot be brought under any kind of government; it breaks all bounds.
Full of deadly poison - He refers here to the tongues of serpents, supposed to be the means of conveying their poison into wounds made by their teeth. Throughout the whole of this poetic and highly declamatory description, St. James must have the tongue of the slanderer, calumniator, backbiter, whisperer, and tale-bearer, particularly in view. Vipers, basilisks; and rattlesnakes are not more dangerous to life, than these are to the peace and reputation of men.

But the tongue can no man tame,.... Either his own, or others; not his own, for the man that has the greatest guard upon himself, his words and actions; yet, what through pride or passion, or one lust or another in his heart, at one time or another, bolts out vain, idle, angry, and sinful words: and he that does not may be set down for a perfect man indeed: nor can he tame or restrain the tongues of others from detraction, calumnies, backbitings, and whisperings; who say, their lips are their own, and who is Lord over us? no man can, by his own power and strength, tame or subdue his tongue, or restrain it from evils it is habituated to, be it lying, cursing, swearing, or what else: God, by his Spirit, power, and grace, can, and often does, change the note of the curser, swearer, liar, and blasphemer; but no man can do this, though he can tame beasts, birds, serpents, and fishes; which shows the tongue to be worse than anything to be found in the whole compass of nature:
it is an unruly evil: an evil it is, for it is a world of iniquity; and an unruly one, being more so than the horse and mule, which are without understanding, who are kept in and governed, and turned any way by the bit and bridle: but though in nature the tongue is fenced by a double fence of the lips and teeth, this is not sufficient to restrain it; it breaks all bounds, and is not to be kept in by nature, art, or argument: nothing but the grace of God can in any measure govern it, or lay an embargo on it:
full of deadly poison, which, privately, secretly, and gradually, destroys the characters, credit, and reputation of men; and is of fatal consequence in families, neighbourhoods, churches, and states.

no man--literally, "no one of men": neither can a man control his neighbor's, nor even his own tongue. Hence the truth of James 3:2 appears.
unruly evil--The Greek, implies that it is at once restless and incapable of restraint. Nay, though nature has hedged it in with a double barrier of the lips and teeth, it bursts from its barriers to assail and ruin men [ESTIUS].
deadly--literally, "death-bearing."

But no man can tame the tongue - Of another; no, nor his own, without peculiar help from God.