Proxy Slur: A term or phrase that is commonly used in a seemingly innocent or neutral manner, but has been assigned a covert, derogatory meaning by specific groups or in certain contexts. The offensive connotation is not apparent to those outside the specific group or context.
"Neighbor": In your example, the word "Neighbor" is a regular term used to describe someone living nearby. However, if two or more parties have agreed to use it as a stand-in for a racial slur, it becomes a proxy slur in their specific interactions.
"Apple": Imagine a scenario where a group uses the word "Apple" to derogatively refer to a person from a certain city or region, unbeknownst to others. In this context, "Apple" functions as a proxy slur.
"Blue Sky": In a hypothetical case, if "Blue Sky" is used among a particular group to negatively refer to a certain ideology or belief system in a covert way, it would be a proxy slur in that context.