Its meaning is how it is most commonly used, right? If so, I think it means some level of a romantic and sexual feeling and attraction. As such, it need not occur immediately, but could develope as a friendship turns into romance, as sometimes happens. The strongest chemistry is probably love at first sight in which an hour after meeting the two find themselves having sex, and planning, or at least wanting, to be together for the foreseeable future. It occurs to me that that particular chemistry becomes less likely as we get older. But I would not want to rule it out.
|