メリクリ is a shortened form of メリー・クリスマス (“Merry Christmas”) which is often used in text messages for brevity. The Japanese seem to be fond of abbreviating words in this manner. For instance, the band “Dreams Come True” (one of my favorites) is rendered in Japanese thus: ドリームズ・カム・トゥルー. This being a pain to say, even for the Japanese, they will often shorten it to ドリカム. Can you see how it was derived from the original?
Another example comes in the form of the Japanese name for a US-based company: マクドナルド (McDonald’s). Perhaps somewhat oddly (to us native English speaker types), the first company president of McDonald’s Japan (藤田田 Fujita Den) decided that the name should be rendered in this mouth-garbling way because it would be more natural for the Japanese to pronounce than how it’s said in English: マクダーナルズ. The word マクドナルド has been known to cripple even the most hearty of native English-speaking learners of Japanese, and can jumble one’s tongue more easily than their hot apple pie. Fortunately, the average Japanese seems to call it マック most of the time, much like we might call it “Micky D’s.” Interestingly, this マック is different from the マク in マクドナルド, but is the same as the マック used in their product names (i.e. ビッグマック, 朝マック, etc.). Additionally, I’ve heard another variation used in the 関西 (Kansai) area that seems to be gaining in popularity: マクド (e.g. マクド行こうけ), which I rather prefer, mostly because speaking in 関西弁 (Kansai dialect) makes me sound like a tough gangster type.
If you can read Japanese well, you might find this poll interesting: http://weekly.freeml.com/chousa/hamburger.html
Anyway, what was I talking about? Oh yeah. メリクリ from Nihongo-a-gogo!