"Mele Kalikimaka" is Hawaiian for "Merry Christmas". Or, more precisely, it's the English phrase "Merry Christmas" as pronounced in Hawaiian. And it was the title of a hit song for Bing Crosby and the Andrews Sisters in 1950:
There's also a (different) 1978 Beach Boys song, originally released as "Kona Coast", which features the same phrase: "Mele Kalikimaka / is Merry Christmas in Hawaii talk-a".
"Wait, what?" you may be asking yourself. "Mele" for "merry", OK — obviously /l/ is the closest thing to /r/ in Hawaiian, we're used to that from stereotypes (and even facts) about Japanese and other varieties of "Engrish". But where did that kalikimaka come from?
Here's the consonant inventory of Hawaiian:
The only fricative is /h/. So what should they use for /s/? Well, according to Allison Adler ("Faithfulness and perception in loanword adaptation: A case study from Hawaiian", Lingua 116(7): 1024-1045, 2006), it's sometimes /h/ and sometimes /k/. Thus the English word crease might be rendered as kaliki or kalihi.
Then there are the extra vowels. That, of course, is because Hawaiian doesn't allow consonant clusters — so that /krɪ/ becomes /kali/ – or syllable-final consonants — so that /mas/ becomes /maka/.
(Actually, according to Adler, the illegal consonants are sometimes dropped and sometimes licensed by adding extra vowels — thus trade might be rendered as /kaleiʔe/ or as /kale:/.)
And you might have noticed that the epenthetic vowels are somewhat variable. Thus Mary Kawena Pukui and Samuel H. Elbert, Hawaiian Dictionary (1986) gives christmas as both kalikimaka and kalikamaka:
But Kalikimaka is apparently the favorite version, and it's the one that made it into the song, as well as into the earliest printed evidence of the borrowed phrase. According to Robert C. Schmitt, "Holidays in Hawai'i", The Hawaiian Journal of History, 29 1995:
The Hawaiian version of "Merry Christmas," Mele Kalikimaka, did not surface until 1904, when it was printed by Ka Nupepa Kuokoa. First proclaimed a national holiday (by Kamehameha IV) in 1862, Christmas was included in the list enacted by the 1896 legislature and has remained a legal holiday to the present time.
("Ka Nupepa Kuokoa" is a Hawaiian-language newspaper that began publishing in 1861.)
So Mele Kalikimaka to all!