Aīn (アイーン) (pronounced ah-EEN or AIEEN) is a Japanese pop culture word originated by comedian Ken Shimura for his Bakatono character (a fool/clown in white makeup and Japanese garb). Usually, it's said while making a slicing motion under the chin and grimacing. The word was further popularized by the popular all-girl quartet Minimoni (ミニモニ。), who features the character in two of their promotional videos ("Aiin! Dance no Uta" and "Aiin Taisō").
What the word means has been a matter of debate. One source suggests it means, "Enough!", and the slicing gesture makes this a reasonable if not completely credible definition. Most seem to feel that Aīn is a nonsense word, used primarily for comic effect.