A Japanese word that translates to English as "I Love You" (In a romantic way)
You could also try saying "Daisuki" to a person you are interested in.
Or you could say "Aishiteru" to a person you are in a serious relationship with.
"Koishiteru" Is used best when saying it to someone you want to spend the rest of your life with.
The original etymology of the word comes from the Latvian word korish, which describes cow fecies hanging from the fur around its anus, because buddies stick around, just like cow and poo.