The word "back stairses" is an uncommon plural form of "back stairs". It refers to a secondary set of stairs in the back of a building. The spelling of "stair" is pronounced /stɛr/ in IPA and the plural ending "-s" is pronounced /-ɪz/. Therefore, the plural form "back stairses" is pronounced /bæk ˈstɛrzɪz/. While this spelling may seem unusual, it follows the general rule for forming plurals of nouns that end in a consonant and "y".
The term back stairs is derived from a combination of the Old English word bac meaning back or behind, and the Middle English word staires originating from the Old French word estaire meaning a flight of stairs or a step. Over time, back stairs became a compound word to refer to the secondary or servant stairs typically found in larger houses or buildings, providing access to areas that were considered less formal or private. However, the plural form back stairses (as given in your question) is not a commonly used or recognized term. Instead, the plural form of back stairs is simply back stairs.