If someone is bi-curious, they are heterosexual — drawn to a different sex — but they also feel some interest in trying a relationship or a sexual experience with a person of the same sex. They are not saying “I am bisexual.” They are saying “I am wondering; I might try.” The bi is the same bi as in bisexual (two / both). The word showed up in newspaper personal ads around 1984 and is still informal: dating profiles, magazines, and talk among friends, not a law or a doctor’s form. Though actually, people stretch it. Some already know they are not only heterosexual and still use bi-curious because it feels softer, less final, than bisexual. Others dislike it, because it can sound as if same-sex feeling is only a test. Use it when someone chooses it for themselves, not as a label you stick on them.