1º_ It is a unisex variant of being up to the balls, which is originally masculine, or of being up to the, which is the female version. In any case the idea is not to put a vertical limit for the tiredness but to use a vulgar word to make the expression more shocking and effective. See ass, be in ass, Quechua/chirisiki. 2º_ The quantity comparison is also used for something that is done too much, such as "deliver to the ass", "vomit to the ass". 3º_ A special case is that of "a lot of distance, remoteness" in which it is used as "up to the ass of [a geographical place]".
