The phrase “ins and outs” dates back to the mid-17th century. It originally alluded to the twisting and turning paths of a winding road, before evolving to describe politicians who are either holding office (in) or out of power. [Dictionary.com]

Over time, these concepts naturally merged to describe the intricate details, complexities, and full workings of a process or subject. [Longman]

What is the origin of the idiom “the ins and outs of something” and when to use it?

The idiom “the ins and outs of something” refers to a comprehensive understanding of a particular subject, including all its details, complexities, and nuances. The origin of this phrase is derived from the idea that “ins” represent the inner or intricate aspects, while “outs” pertain to the outer or surface aspects of a subject or situation. Together, it signifies a thorough knowledge of a topic, situation, or process. This phrase is used to convey that someone possesses a deep and comprehensive understanding of a subject or can navigate through complex situations with ease, having explored all facets of it. https://dictionary.langeek.co/en/word/214128?entry=ins%20and%20outs

Excerpt from Chapter Thirteen:

You are not yet of age,” he declared. “You were born in England to English parents. Such makes you an English citizen, who has been living abroad,” he reasoned as he cracked the door a bit wider so she might view him and his obvious determination. “As such, with your father’s passing, you became the ward of either the Kirkland earldom or the Summerville baronetcy. I have drafted a letter to Sir Jasper to inform him of my plan to bring your mother and your brother to England and ask for his and Lady Anna’s support in returning our joint family to English shores. As Grayson is to inherit the Kirkland earldom, I believe he should stay at my home estate. At his age, he should learn the ins and outs of the estate, as well as his responsibility to the earldom.”

“In truth, Grandfather, I doubt Grayson has considered all you mentioned. Despite his age, he still heavily depends on mother’s opinions. If she chose to return to her parents’ home, I would expect Grayson would follow.” 

Her grandfather did not speak for several minutes. “You will stay in Derbyshire, will you not, Christiana? I have so many things I wish to show you.” 

“I can think of no place I would rather be, Grandfather, but, if this plan by Lord Liverpool proves faulty, I would rather be in a different part of England, to assure both my well being and yours. If the marriage contract is ruled as null, Lord Whitmorris will come for me again and, the next time, I could end up in the same condition as his lordship’s man of business.  I shan’t place you or the others in danger. Lord Whitmorris is not one to forgive and forget.” 

“Then we see that Lord Whitmorris does not know success. When the Lowerys called on me and found me in despair, I made myself a promise not to permit Whitmorris to win. You were my hope. You were in England, and I finally had an opportunity to look upon you again. As I dressed for our reunion, I stopped to speak to your grandmother’s portrait to tell her what I planned. I told Christine that I . . .” 

“Christine?” She asked. “I did not realize father’s mother was a Christine. Surely he must have said so when I was young, but I cannot recall his doing so. Was I named for her?” 

Her grandfather’s voice sounded as if it was watery with his tears. “You were.” 

“Why did no one tell me? I have lost—my family has lost—so much of our history.” 

His lordship cleared his voice. “Your parents insisted on naming you after both your grandmothers. You are ‘Christiana’ after my wife ‘Christine’ and Sir Jasper’s ‘Lady Anna.’ We all agreed that as the first born you should have the name of both your grandmothers. It was hoped there would be multiple sons to carry the other names. Yours, as the first born, tied us together forever. Unfortunately, after the death of Mr. Greenville, the Pavlicks feared the wrath of others in society, especially after the accusations against your father became known. They claimed their distance from me and from the earldom. It would have been my son’s wish for both them and me to know a clearing of his name.” 

“My wish also,” Christiana whispered through her own tears. “I want a full and loud family—no more hiding in shame. “