to stick fast to something and not let go: to cling, hold tight, or stay glued
At its root this word describes one thing sticking fast to another so that the two will not come apart: a tongue glued to the roof of a dry mouth, scales locked together, a hand frozen onto a sword. Every other use grows out of that single physical picture, which is why one plain word can stretch so far.
From sticking, it moves to people. To cling to someone is to attach yourself to them and refuse to be pried away, whether out of love, loyalty, or need. That is the word for a husband joined to his wife and for Ruth refusing to leave Naomi.
The Bible's deepest use turns that loyalty toward God: to hold fast to the LORD is to stay attached to him with the same stubborn closeness, the opposite of drifting off after other gods. The same closeness can be dangerous, though. A pursuer who sticks to his quarry catches and destroys it, and a curse, a disease, or a sin can cling to a person just as tightly as devotion does.