`Johnny' was applied as a nickname for Confederate soldiers by the Federal soldiers in the American Civil War; `greyback' derived from their grey Confederate uniforms
"The Federal troops mockingly called the Confederates Johnny while referring to their distinctive grey uniforms as greys."
A jack (the playing card)
"I need to buy more johnnys before our trip tomorrow."
A condom.
A diminutive of the male given name John.
"Johnny is a common nickname that people use when they want to address someone named John in a friendly way."
Johnny comes from adding the suffix "-y" to the name John. It was used as a term for an Englishman, similar to how "John Bull" refers to a typical British person.