"Mele Kalikimaka is the thing to say / On a bright Hawaiian Christmas day"
What makes Bing Crosby's classic Christmas tune (well, the other one anyway) so endearing? At least part of the appeal is "Mele Kalikimaka" itself, which sounds tantalizingly close to "Merry Christmas" and yet not quite the same. So how did the Hawaiian language end up with this phrase?
Simply put, Hawaiian borrowed the English "Merry Christmas," which speakers then adapted to fit the sounds of their language. In the video below, YouTuber Tom Scott explains how this process happened:
You can also think about English Christmas itself. Although the spelling hasn't changed, we've worn down the pronunciation a bit over the years from christ+mass to criss-miss: English vowels frequently morph into schwas, and when was the last time you heard someone actually pronounce that T?
Even with the T, of course, the Christ part of Christmas isn't Jesus' last name. Rather it's a title that comes from the Latin Christus—itself from the Greek khristos (χριστος) meaning "the anointed"—a literal translation of the Hebrew mashiah (also the source of messiah). The "kh" in khristos represents the Greek letter chi, written X, which in Ancient Greek sounded a lot like, well, a K followed by an H (as in a careful pronunciation of steakhouse or backhand). Latin didn't use the letter K, so the Romans wrote it as "C+H" instead. But people pretty quickly started ignoring the H since there weren't any domestic Latin words with that K/C+H sound. Unless you knew Greek, you didn't really know what to do with words like chorus, chaos, or Achilles, and you just reverted to a hard C.
In fact, we narrowly escaped having the same confusion over the spelling of Christmas as we do over the spelling of Hanukkah (or is it Chanukkah?). The sound at the beginning of Hanukkah/Chanukkah isn't particularly common in English, but you may have encountered it in Scottish loch or German Bach. To some people, it sounds like a breathier /k/ (hence the C+H, which this time indicates breathiness), while to others it just sounds like a variation of /h/ itself. So Chanukkah and Hanukkah represent two different ways people have tried to explain this sound to English speakers. Neither are completely successful, because it's just plain hard to produce a sound that you're not accustomed to—which is why Latin ended up with the /k/ sound for "ch" and Hawaiian ended up with Mele Kalikimaka.
Back to Christmas. Although the Ancient Greeks pronounced the kh in khristos as K+H, they—some centuries later—eventually started pronouncing all instances of the letter chi as that same breathy sound found in loch, Bach, or Chanukkah/Hanukkah. So if we'd borrowed khristos at a much later date, and not via Latin, we might have ended up trying to write it phonetically using h—Hristmas!—which is in fact what Modern Greek speakers often end up doing when they're writing informally in our alphabet. But we didn't, so we don't.
And the fact that Greek chi is written X explains yet another Christmas linguistic mystery: why Christmas is abbreviated Xmas. It's originally Chi-mas, but English speakers don't pronounce it that way because of a further orthographic complication. When the Romans were borrowing and adapting the Greek alphabet, they managed to borrow the symbol X from a group of Greeks who used it to stand for the sound /ks/, despite the fact that the majority of Greeks—who they later borrowed lots of words from, including christus—used X to stand for that K+H sound.
The really inexplicable thing is why the Romans bothered borrowing anything to stand for /ks/, when they could have just written it with two letters, like they did with psi ψ as in psychology. But not to worry. As far as I know, Bing Crosby never wrote a song about how to say Merry Christmas in Ancient Greek.