I met a lot of hard-boiled eggs in my life...

[Sgt_Markoff]

What is, "hard-boiled"? Discuss.

[TopBilled]

Yeah, this term never made sense to me. Usually when things are boiled, they soften.

[LawrenceA]

The phrase 'hard-boiled' itself in respect to human character originates somewhere in the last quarter of the 19th century. It is said to originate from the notion that a hard-boiled egg is tough, or 'hard to beat' as a vaudeville gag would have it (with reservations read Peter Tamony.) The hard-boiled egg as a measure of human character is prefigured by the 'bad egg' which is often said to have its origin in the Samuel Hammett novel Captain Priest (1855.)

https://www.quora.com/What-is-the-etymology-of-the-term-hard-boiled-fiction-detective-etc

[Sgt_Markoff]

The "boiled shirt" was an aspect of life in the late 1800s. You would wear it to church, to lynchings, bull-and-bear fights, etc.

https://en.wikipedia.org/wiki/Dress_shirt#Formal_shirts