to forget: to lose hold of something, whether from memory or from active care
At its core this word is about losing hold of something that was once kept. In ordinary use that loss is simply mental: a name slips away, a kindness goes unremembered, a fact drops out of mind.
But the Bible's most weighty use of the word is not mental at all. To forget the LORD, his covenant, or his commands is not to have a memory lapse; it is to stop paying attention and to walk away from him. Forgetting and abandoning are the same act here, which is why the prophets treat forgetting God as a betrayal rather than an accident. The opposite is not merely recalling a fact but keeping faith and staying loyal.
The word also runs the other direction. When God is said to forget his people, or when a sufferer pleads that God has forgotten him, the issue is not divine memory but divine care: to be forgotten by God is to be left without help. Isaiah answers that fear directly, insisting that a nursing mother might sooner forget her child than God forget his people.
In its passive form the word means to be forgotten, to drop out of human memory entirely, the fate the wise and the fool finally share.