Actually the granddaughter of Omri, since Athaliah was the daughter of Ahab (ch. 21:6), who was the son of Omri. Omri is mentioned instead of Ahab because Omri was the founder of the house. On the use of son as grandson see on 1 Chron. 2:7.

 This figure is given as 22 in 2 Kings 8:26. Ahaziah could not have been 42 years old when he took the throne, because his father died at 40 (2 Chron. 21:5, 20). Two explanations of this have been offered. One is that an error in transcription is responsible for the word “forty” instead of “twenty.” The other is that the Hebrew phrase “a son of 42 years” refers not to Ahaziah’s age at his accession, but to the number of years from the founding of the dynasty of Omri, since Ahaziah was a “son” of that dynasty through “Athaliah the daughter of [Ahab, son of] Omri.” That Ahaziah was under the tutelage of that royal house is evident from vs. 3-5 and from 2 Kings 8:27, where he is called a “son in law of the house of Ahab.” It should not be surprising to find a fragmentary reference to an era reckoning from Omri, since Omri was so important a ruler that other nations continued long after to call the land of Israel the land of Omri, and the kings of Israel sons of Omri (see on 2 Kings 8:26). From the beginning of Omri’s reign to the accession of Ahaziah was about 42 years.