doing something in a way that somehow hinders and irritates

"Standing on someone's toes" just means that your foot is on top of theirs. "Stepping on someone's toes," however, refers to the idea that you are stepping in on their territory and taking over something that belongs to them (either in reality, or just in their minds."

The image is of someone stepping onto another person's feet. Figuratively, it means overstepping your bounds, doing something that is actually someone else's job.

While it is highly unlikely that a human being would be born with thirteen toes it is possible someone could have a birth defect which could result in having 13 toes.

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