The phrase "come outer" does not have a specific etymology on its own since it is a combination of two separate words, "come" and "outer". However, if you are referring to the term "come outer" in the context of British dialect, it is a regional variant and contraction of the phrase "come out of it", which means to stop being foolish or delusional. The origins of this phrase date back to the early 19th century, but the exact etymology is uncertain. It is likely to have evolved from colloquial expressions used in various dialects across the UK.