A waiting opponent who has assumed a defensive posture or expression is often weak.
Similar to how a waiting-for-action opponent who doesn’t want you to bet will stare at you, some players will seem physically ready to “go to war” with you. They might look at you, turn toward you, lean toward you, grab their chips, sit up straighter in their chair, scowl at you, or assume a number of other defensive poses. Their bodies will get tense and their faces will look more tough or angry, as if preparing for a confrontation.
Defensive body postures are only likely to be displayed when a player is sitting directly beside you. This is just because a player can’t try to physically intimidate you when they’re far away. They can stare daggers at you across the table, but they can’t act out aggressive body language unless they’re very close to you.
If I asked you to look tough or angry, what facial expressions come to mind? One way to do this would be to harden your lips, making them smaller and slightly jutting, and to lower your eyebrows. Your opponents will sometimes unconsciously assume varying degrees of this expression when they are weak and trying to discourage a bet.1
1A note: Be sure to look for the furrowed brow (which can be subtle) that goes along with the pursed lips. This is because lightly pursed lips by themselves can mean something completely opposite. Try this: raise your eyebrows and purse your lips. What does that feel like? It might feel like you’re trying to hide something or get away with something. It’s possible that if you witness the pursed lips without the lowered eyebrows you are actually seeing a player with a good hand trying to be coy.
This tell can be used similarly to the grabbing-chips tell. A player performing this tell may have a hand he will call with, but he is most probably in defensive mode and would prefer a check. Almost certainly he will not be raising you. In a no-limit situation, if I had a weak hand and I spotted a player acting this way on the flop, I would probably bet into him, thinking a call is possible. If he calls, and has the same posture on the turn (and assuming the player isn’t a calling station), I’d bet again, expecting a fold.
Spotting this tell can also be useful for making thin value bets. If you spot this tell on the river, you can bet some hands you might usually check.