The phrase “If it’s yellow, let it mellow” is a colloquial expression often used to suggest that if urine is yellow (indicating it’s not too concentrated), it can be left in the toilet and not flushed immediately. This saying is typically part of a broader conversation about water conservation and reducing the number of times toilets are flushed.
The idea behind this saying is that flushing toilets uses a significant amount of water, and by not flushing every time, one can save water. This practice is sometimes promoted in environmental discussions as a way to reduce water waste in households.
In a humorous or informal context, the phrase highlights a relaxed attitude towards bathroom etiquette, though it might not be universally accepted or appreciated in all social settings.