A slang term used to describe any skill or ability that gives you limited control over when you expend it. The most common examples are Defenses, which activate upon the "first" attack/effect of a given type that you are subjected to.
They differ from Smart skills/abilities that allow the player to more strategically time things and choose when to expend them.
It is possible to strategically use Smart skills/abilities to reserve a dumb skill/ability for a later time, rather than having to expend it at that moment. Using your smart and dumb defenses in this way requires a good grasp of the game rules and your skills/abilities in order to do it properly. This type of strategy is best used once you're a more seasoned player.
For example: Pretend you had a Dumb defense against ALL mental attacks, and a Smart defense specifically against Fear attacks. You know that there's going to be a big terrible fight coming that you've heard uses a lot of varying mental attacks that are especially dangerous for that encounter. But before you get to that fight, you get randomly ambushed by some puny goblins on the road who use a Repel by Fear. It is possible to use the smart Fear-specific defense first. This keeps you from losing the Dumb defense against all mental attacks, which will serve you well during that upcoming fight you heard about.