How should this expression be used, and what is its origin?
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It is an interjection with literal meaning "look and see". It is used to demonstrate surprise.
According to the Merriam-Webster dictionary, the first known use was in 1808.
As for the etymology, according to the Wiktionary, "lo originated from the shortening of the word loke, commonly seen in Middle English texts".
Some interesting examples:
Lo comes from Middle English, where it was a short form of lok, imperative of loken, "to look" (see Etymonline, Wiktionary). To behold means "to see, to look at" and comes from Old English bihaldan, "give regard to, hold in view" (compare to behalten in contemporary German). So the literal meaning of the phrase is "Look and see!", but nowadays it is used as a set phrase and an interjection to express surprise. Merriam-Webster says that the first recorded appearance is from 1808. Wiktionary adds that it is "often used ironically or humorously", and the Cambridge Idioms Dictionary marks it as "humorous".
Origin, from wiktionary:
It sounds archaic and a bit strange, so use it to get a kind of like.. overly formal/surprised effect.