"Then why do I have a strange clammy feeling down my spine?"
The Willies is a recurring element in the show Jackie Chan Adventures. It happens when a character senses an oncoming danger which causes them to experience a sudden, overwhelming tingle.
Uncle often has the Willies in relation to Daolon Wong.
This first happens when Wong prepares to attack Tohru when its believed he is the chosen one, Uncle is woken from his sleep and informs Tohru that he has the willies due to the dark chi. Uncle also gets the Willies when he senses Daolon Wong hiding among the Three Monkey Statues, hiding among the woods, and when he comes after them for the Noble Yak.
In season 4, Uncle even experienced the Willies when he realized he didn't have the keys to his shop.
It appears several times in season four. Tohru mainly experiences it in relation to the Oni Masks. It could come as a warning sign that Tarakudo was in the same room spying on them in he room. This was one of the factors that convinced Uncle that Tohru should take his place in the fight against Tarakudo and that it'd be better if he returned to San Francisco. However, Uncle changed his mind when he experienced the Willies himself which made him realize that he didn't have the keys to his shop. Uncle even uses Tohru's willies to conclude that there are no more Oni Masks.
In an ironic twist, Tarakudo himself has the Willies when he thinks that he's successfully seizing the world, letting him know that the Chans are up to something.