The phrase "permanent stain" refers to a mark or blemish that cannot be removed or undone. It suggests that something negative or damaging has happened and its effects will last forever. Full definition
Breast milk is digested quicker than cow's milk or formula, is less likely to come back up, and doesn't leave permanent stains on clothes. (drbenkim.com)
If you are concerned about permanent stains or damage, protect the areas of your house that cause concern. (verywellfamily.com)
Many of these are slightly acidic in nature, and if they spill and splatter on hardwood, they can eat away at the finish or cause permanent staining in the wood. (thespruce.com)