Compare Proverbs 17:21. Here is added a reference to the sorrow which the folly of a child brings especially to the mother.

A foolish son is a grief to his father,.... Because of his folly and wickedness, and the ruin he is bringing himself to;
and bitterness to her that bare him; a cause of bitterness of soul to his mother, more distressing than the bitter pains with which she brought him forth into the world. Jarchi, by the father, understands the blessed God; and by her that bare him, the congregation of Israel; to whom Jeroboam, the son of Nebat, was bitterness, who caused Israel to sin; see Proverbs 10:1.

Wicked children despise the authority of their father, and the tenderness of their mother.

a grief--or cross, vexation (compare Proverbs 17:21; Proverbs 10:1).

The series of proverbs, v. 25-18:2, begins and closes in the same way as the preceding, and only Proverbs 17:26 stands by itself without apparent connection.
This verse begins connecting itself with Proverbs 17:21 :
A grief to his father is a foolish son,
And a bitter woe for her that bare him.
The ἅπ. λεγ. ממר is formed from מרר (to be bitter, properly harsh), as מכס from כּסס. The Syr. and Targ. change the subst. into participles; some codd. also have ממר (after the forms מחל, מסב, מפר, מרע), but as may be expected in 25a, מבעיס. The dat. obj. instead of the accus. may be possible; the verse immediately following furnishes a sufficient example of this.