Compare the marginal reference. Since nothing is nobler than the self-sacrifice of the true friend Proverbs 17:17, so nothing is more contemptible than the weakness which allows itself to be sacrificed for the sake of worthless associates.
In the presence of his friend - i. e., "On behalf of" or "to his friend for some third person."

Striketh hands - Striking each other's hands, or shaking hands, was anciently the form in concluding a contract. See notes on Proverbs 6:1.

A man void of understanding (i) striketh hands, [and] becometh surety in the presence of his friend.
(i) Read (Proverbs 6:1).

A man void of understanding striketh hands,.... With his friend's creditor, and becomes surety for him; and thereby acts a very unwise part, and shows himself to want understanding, by taking such a step, which may prove the ruin of himself and family: for though a man may and should love his friend at all times; yet he is not obliged, under a notion of friendship, to injure himself and his family, or to run the risk of it; if he does, it is a plain case he wants wisdom and discretion, see Proverbs 6:1;
and becometh surety in the presence of his friend; not the creditor, but the debtor; and to pass his word for him, when he is present, shows that his own word will not be taken; and that he is either thought to be in bad circumstances, and incapable of payment at the proper time; or else that he is a bad man, of dishonest principles, and will not; and in either case it is not advisable to become surety for such a man: and besides, doing it in his presence may make him more careless and unconcerned about making good his payment or contract at the appointed time, when he knows his friend is engaged him.

Let not any wrong their families. Yet Christ's becoming Surety for men, was a glorious display of Divine wisdom; for he was able to discharge the bond.

(Compare Proverbs 6:1-5; Proverbs 11:15).
in the presence, &c.--that is, he either fails to consult his friend, or to follow his advice.

18 A man void of understanding is he who striketh hands,
Who becometh surety with his neighbour.
Cf. Proverbs 6:1-5, where the warning against suretyship is given at large, and the reasons for it are adduced. It is incorrect to translate (Gesen., Hitzig, and others) לפני רעהוּ, with the lxx, Jerome, the Syr., Targ., and Luther, "for his neighbour;" to become surety for any one is ערב ל, Proverbs 6:1, or, with the object. accus. Proverbs 11:15, another suitable prep. is בּעד; but לפני never means pro (ὑπέρ), for at 1-Samuel 1:16 it means "to the person," and 2-Samuel 3:31, "before Abner's corpse (bier)." רעהוּ is thus here the person with whom the suretyship is entered into; he can be called the רע of him who gives bail, so much the more as the reception of the bail supposes that both are well known to each other. Here also Fleischer rightly translates: apud alterum (sc. creditorem pro debitore).

Friend - Debtor, for whom being his friend, he becomes surety.