Used to refer to something or someone random, or of no particular importance, or even someone or something you might dislike.

The 'ass' part has no meaningful contribution and is therefore a random ass addition.

Adj. Something that is expected to have a long lifespan by virtue of having existed for a long time, based on the lindy effect, named for the New York restaurant Lindy's, coined by Albert Goldman and popularized by Nassim Taleb.