expression, godan verb (-su)
disgrace one's ancestors; tarnish the family honor
Idiomatic expression literally meaning 'to defile the ancestral tablets'. Used when someone's actions bring shame upon their family or ancestors.
犯罪を犯して位牌を汚すようなことはするな。
Don't do anything that would disgrace your ancestors, like committing a crime.
From 位牌 (ancestral tablet) + を + 汚す (to defile). The expression metaphorically extends the act of physically defiling a sacred ancestral tablet to any action that brings shame upon one's lineage.