Albert Barnes Commentary 2 Kings 11:2

"But Jehosheba, the daughter of king Joram, sister of Ahaziah, took Joash the son of Ahaziah, and stole him away from among the king`s sons that were slain, even him and his nurse, [and put them] in the bedchamber; and they hid him from Athaliah, so that he was not slain;" — 2 Kings 11:2 (ASV)

Jehosheba... sister of Ahaziah — According to Josephus, she was his half-sister: the daughter of Joram, not by Athaliah, but by another wife. She was married to Jehoiada the high priest and was thus in a position to save and conceal her nephew, Joash, who was only one year old (Compare to 2 Kings 11:3, 21).

In the bedchamber — This literally means “in the chamber of mattresses,” probably a storeroom in the palace where mattresses were kept.