Now anyone reading this article - http://mentalfloss.com/article/31841/why-new-york-city-called-big-apple and especially this line from that: "The Big Apple. The dream of every lad that ever threw a leg over a thoroughbred and the goal of all horsemen. There's only one Big Apple. That's New York." can say that it talking about a boy or lad that ever rode a horse, then why is it using the Idiom "throw a leg over" which when googled, shows to mean "to have sex with" ?
While we're at it, is there a website or reference link that has the detailed meaning and usage information on such idioms(especially US English Idioms and Proverbs) which give an indication of something, but when used in different contexts(and situations), means a variety of different and un-related things or situations ?
Expecting(if possible) a brief explanation with history when such Idioms may have started to divert their meaning in different direction than what they used to mean when used in British-English.