English StackExchange has a thread about this, though I’m not seeing a lot of sources. According to this website, looking in (the) mirror has been used since at least the 15th century.
I think the phrase is just a side effect of a less common use of the definite article “the”, as the last answer on SE suggests.
According to Merriam-Webster, one of the uses of “the” can be:
l —used as a function word before the name of a commodity or any familiar appurtenance of daily life to indicate reference to the individual thing, part, or supply thought of as at hand
talked on the telephone
I suppose it’s just an idiomatic way of using “the”. I can’t find any information about why this usage has developed, but other languages similar to English do the same, so it could just be a Germanic or Latin language quirk?
English StackExchange has a thread about this, though I’m not seeing a lot of sources. According to this website, looking in (the) mirror has been used since at least the 15th century.
I think the phrase is just a side effect of a less common use of the definite article “the”, as the last answer on SE suggests.
According to Merriam-Webster, one of the uses of “the” can be:
I suppose it’s just an idiomatic way of using “the”. I can’t find any information about why this usage has developed, but other languages similar to English do the same, so it could just be a Germanic or Latin language quirk?