Kihachiro Onitsuka was a Japanese soldier who after WWII decided that sports would be good for Japanese youth. He therefore learned how to make shoes circa 1949. He then founded Onitsuka Co Ltd. In 1958 he bought the "Tiger" brand. In 1972 they merged with another company called Jelenk and then they decided they needed a new name. So,they found an old latin phrase that said, "Anima Sana In Corpore Sano"; which means, "A soundmind in a sound body". By taking the first letters of each word in the latin phrase you get ASICS, which is the name of the shoe brand.
Now a days Onitsuka's are used alot by the "scene" or "scenester" subcultures, specifically those consisting of indie and hipster styles.
Often confused with love, this is the feeling that a particular woman is actually special. This is just an illusion; she is the same as the other three or so billion. "Go fuck ten other women" is the most commonly prescribed treatment for this "disease" (hence the "itis"), as it tends to show quite quickly how very alike people are.
In other words, get over yourself and your obsession with that girl, because it's just an illusion. And when you get rejected, don't be depressed. Because there's really not much to worry about.
Dude, you definitely have oneitis for that girl. You've been talking about her nonstop for the past week. Don't you realize she's nothing special, and that you are just being fooled? Snap out of it so you won't be so depressed when she rejects you!
There is only one simple explanation for the term: An unhealthy romantic obsession or fixation for/on one person. This mental disease will often interfere with your ability to attract said person, due to your clingyness.