This expression or saying comes from 2 possible sources, but in the meaning “the law is an ass” -the ‘ass’ in this instance, is a donkey, which has a reputation for being stubborn, and obstinate, without reason. So to define the expression, is to say that the application of the law is often contrary to common sense.
Most ‘functioning societies’ have a system of laws, which are drafted for, and against the citizens of said societies. At the heart of this system is the ‘lawyer’, ‘attorney’ or ‘solicitor’ (UK). I believe that all three are essentially the same( please correct me if I am wrong). Now, depending on which country one resides in, lawyers ( I will use lawyers so as not to confuse our American friends), are either: despised, or revered, mainly for contradictory reasons, ie; the possibility of earning large sums of money(revered/despised), or manipulation of people, or the law, to obtain favourable results( again, revered or despised).
Whether you are in the for or against corner, one thing is beyond question. The law itself is 
complicated and at times lacks common sense. We have complex legal structures and systems which require several years of intense study and training, after which, working within the system is also required to fully understand the complexities of it all. Dealing with criminal law, which is often the only interface with regular people such as myself, there is normally a system of ; judge, lawyers, witnesses, and a jury, in its most basic form.
Now the judge usually does not get to be in this position of authority until well into their careers, often after 20 plus years or more. This is to ensure that they have the longevity, and experience, and tried and tested decision making skills. After all, peoples lives are in their hands. Lawyers to a lesser extent, also need to have longevity and decision making skills in the same environment, due to the fact that peoples lives are also in their hands, but they often do not have the same decision making power.
So why do we so often base this system( or the guilt or innocence of people) on a series of “yes or no” questions, which do not seem to be susceptible to any deviation or any other realms of possibility or probability. Then, after dissecting through all the intricacies of the cases put front of them, using this ‘ basic’ system, do we then drag 12 people off the street, with ZERO legal experience, and no skill at sifting through evidence correctly, or efficiently. Who are also asked to listen to weeks of legal-speak, which most of us (conveniently, for the system) don’t understand. Then finally after listening to lawyers appealing to their ‘human side’, they have to make decisions, using their non-emotional, non-human skills, to deliver decisions which are basically the power of life or death over us, the general public.
This is one area where the law appears to be ‘an ass’, namely due to the fact that if everything else in the system is so complicated and abstract, such as from arrest, to trial, to summations, then why is the final part of the process so simplistic and placed in the hands of ‘laymen?’
Is there no thought there, or Is it that the legal system wants to absolve itself of all responsibility for
‘locking people up, or executing them’ that it transfers this responsibility on to us mere mortals. This way they can say “we don’t decide who is innocent or guilty” but in essence, they do, because they designed the system. So deep rooted are these institutions in our psyche, that change is not only unwanted (by them), but also impossible, (because of them) without a complete about-face in our attitudes to the way we treat each other. The people who do all the ‘leg work’ in the legal process, and also those who are rewarded the most, should ultimately use those years of training and experience to decide the fate of the people who come up against the very same system. Without these changes, and more I could mention, the law will continue to be “an ass for the foreseeable future.
