I was taught as “shumoku ashi”, not shimoku ashi.
However, according to Japanese dictionary, they are both OK. Shimoku looks like a dialect.
Either way, shumoku is a hammer to hit a huge bell at temple.
Shumoku Ashi is to express how monks stand when they hit a bell with a hammer.
In Shumoku Ashi, the feet of monks are angled.
The front foot may be straight but the back foot is totally facing sideways.
Interestingly, according to a Japanese website about kendo, shumoku ashi is the way Japanese flight attendants stand.
The front foot is facing straight and the back foot is facing aside.
That is shumoku ashi.
It is considered to be bad because the back foot in kendo should not face sideways. So if you hear that you have shumoku ashi (shimoku ashi), your back foot is facing outwards rather than facing straight forwards.
Hope this helps.