Glossary
Dry peek
In one line: Peeking with no utility behind it: no flash, no smoke, no information.
What it means
A dry peek is stepping out of cover "dry": just you, your gun and the hope of winning the duel. Sometimes it is the right call (the enemy does not expect it, you have a positional edge, there is no time), but often it is a gift to whoever is waiting with their crosshair ready.
The practical rule: if you have a flash, a drone, a smoke or a teammate who can help, use them first. Repeatedly dry peeking the same angle is one of the most common mistakes in the lower tiers.
An example
A defender is holding the angle at the end of main. Instead of dry peeking, you ask your initiator for a flash and swing when they are blind. The same duel, a coin flip when dry, is now almost a sure thing.