孝、孝顺，孝道、孝敬 all talk of filial piety which is a huge part of Chinese language and culture.
愚孝 on the other hand means "stupidly filial" and is defined by Taiwans Ministry of Education as, “昧於事理的孝行，如割股醫親之類。” which roughly translates as “filial acts without judgement, things such as cutting off the flesh from one's thigh to make a medication for one's sick parent."
abc dictionary defines 愚孝 as, “blind devotion to one's parents,” while cc-cedict says, “unquestioning filial piety.”
Years ago there was a compilation of works called “二十四孝” (the 24 examples of filial piety (selected by Guo Jujing of the Yuán 元 Dynasty) but many of these, so-called, “exemplars” are considered 愚孝 by today's standards, see:
However, in modern times, some stories in The Twenty-four Filial Exemplars are regarded as examples of blind filial piety that should not be learnt from. These stories include: Guo Ju burying his son for the sake of his mother; Wu Meng allowing mosquitoes to suck his blood, believing that they would not bother his parents if he did so; Wang Xiang lying naked on ice in the hope of thawing the ice with his body heat so that he could catch the fish beneath the ice.
There are also some stories which were heavily criticised and even regarded as contradicting Confucian principles. One example is the story of Cai Shun being rewarded by the Chimei rebels for his filial piety: The story paints the rebels in a positive light when they were actually being disloyal to their country by rebelling against the government. Another example is the story of Laolaizi behaving in a childish manner to amuse his parents: The writer Lu Xun mentioned that Laolaizi's story is an insult to the ancients, and has a bad influence on future generations.
So, The Questions:
What was 孝 originally meant to mean?
At what point does 孝 stop being 孝 and become 愚孝?
What does 孝 mean today?
What does 愚孝 mean today?