to slip: to lose one's footing, so that what was holding a person up gives way
The picture behind this word is always the same: the ground stops holding. A foot slides, an ankle turns, a step goes out from under someone. Hebrew has separate words for tripping and for falling; this one names the loss of grip itself, the moment when a person's footing fails.
Most of its uses are denials, and that is where its force lies. A rescued king says God widened the ground beneath him so that his ankles never gave way (2 Samuel 22:37, and the same song again at Psalm 18:36). A worshipper who has put his trust in God says simply, "I shall not slip" (Psalm 26:1). A man with God's instruction in his heart walks without his steps sliding out from under him (Psalm 37:31). In each case the claim is not that the person is strong but that the ground under him is being held steady.
The remaining uses turn the picture over. Proverbs 25:19 says that leaning on a treacherous person in a crisis is like chewing with a bad tooth or walking on a foot that gives way: the very thing you put your weight on is what fails you. Job 12:5 pictures the comfortable looking down on someone whose foot is already sliding. And Psalm 69:23 asks God to make an enemy's hips give way for good, so that they cannot stand at all.
Two nearby words are worth keeping apart from it. The Hebrew verb for being shaken or dislodged from a fixed place (מוֹט) describes something firmly set that gets moved, and the verb for stumbling and going down (כָּשַׁל) usually involves weakness, exhaustion, or defeat. This word sits between them, on the treachery of the footing itself.