one way

This is a Counter-Strike: Global Offensive (Csgo) term.
In Csgo there is a utility grenade that gives a competitive advantage when used correctly, called a smoke grenade, or smoke for short. This term revolves around the usage of the smoke grenade.
An "one way" is refering to a cleverly placed smoke grenade that allows only one direction of vision. In other words: "I see you, but you can't see me."
This happens commonly in Csgo because the game is extremely competitive and any advantage will cause a victory.
The term became popular because it's a quick way to alert your team of the threat, because an one way smoke will cause massive losses to the enemy.

"Did you smoke snipers nest?"
"NO, it's a one way, WATCHOUT."
Note: Snipers nest is a common one way position in csgo, watchout.