He had decided to call her Hedwig, a name he had found in A History of Magic. (Harry Potter and the Philosopher's Stone, Chapter 6)
Is there any canon info on who that Hedwig was in-universe? HP Lexicon mentions that it was a out-of-universe saint.
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During an interview that she conducted in 2000, JK Rowling identified the root of many names used in the HP canon;
How do you come up with all the unique names, places, and things that help make Harry Potter so intriguing?
Many of the names are invented, for example "Quidditch" and "Muggle." I also collect unusual names, and I take them from all sorts of different places. "Hedwig" was a saint, "Dumbledore" is an old English word for "bumblebee," and "Snape" is the name of a place in England.
Saint Hedwig (in addition to being the Patron Saint of Silesia) is famed for being the Patron Saint of orphans, having opened several orphanages in Poland during the 13th Century.
In-universe, there's no canon explanation why "Saint Hedwig" would be included the "History of Magic" (possibly a famous half-blood wizard or witch grew up in one of the literally hundreds of St Hedwig's monasteries and orphanages around the world) but it's hardly surprising that Harry would gravitate toward the name of someone famed for caring for people who'd lost their parents.