The Cog-Stopper shirt is a shirt that is available to Toons during Operation: Storm Sellbot Headquarters.
Obtaining the Cog-Stopper Shirt
To obtain the shirt a Toon would have to complete the Toontask for the Cog-Crusher outfit first (Which is given by Lord Lowden Clear. When finishing that task if the Toon talks to Lowden again, he will have another task for the Toon. After finishing the task, Detective Lima will give you the shirt and send it to your mail at your Estate.
A toon outside of Sellbot HQ with her Cog-Stopper Shirt