"Slippery when wet" is a red text properties modifier on SMGs in Borderlands 2
Found on the following red text Sub-Machine Gun: Crit, such as the Consummate Crit from Tiny Tina's Assault on Dragon Keep DLC
Grants Weapon Effects
Always shock damage
High crit dmg
Chance to fall out of your hand (your character will auto-switch to a different weapon)