I saw the Wiki page of Inherent Vice, I did't find much info about the title.
Inherent Vice was adapted by Anderson from the novel of the same name by Thomas Pynchon.
Why it is called "Inherent Vice"?
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“Inherent vice,” as a term in commercial law, refers to the fact that in any contract or transaction there are forms of risk that cannot be avoided. Ship eggs by road or rail, and some of them will get broken.
But it also has a broader meaning connoting the fundamental instability of physical objects, and is used in that sense by librarians and archivists: Books and manuscripts eventually degrade and crumble into dust; as a film buff like Anderson surely knows, old motion-picture prints on cellulose acetate melt away into vinegar-scented glop. (Something like 90 percent of the films made before 1920 have been lost.)
Both kinds of inherent vice are on display in “Inherent Vice,” one might say; Doc Sportello’s attempt to connect the threads of nefarious conspiracy beneath the surface of 1970 Los Angeles is undermined both by the flawed nature of the universe and by the sorts of things that go wrong in any human endeavor driven by lust and greed.
The voice-over by Sortilège at the 1 hour, 51 minute 39 second mark states:
"Inherent Vice, in a marine insurance policy, is anything you can't avoid. Eggs break, chocolate melts, glass shatters, and Doc wondered what the meant when it applied to ex-old ladies..."
There's two parts to why this is relevant:
Doc is clearly, completely, and throughout the film obviously hung up on his break-up with Shasta. He spends a good chunk of the film looking for her, and after their...encounter...on his couch he denies that they are back together. It was all passion and frustration, it was her egging him on ("If I had the faithless little bitch over my lap, like this..."). The film is, in big part, about inevitability of things, especially how we feel about past relationships.
The reason there is a discussion about marine law has to do with the shell necklace worn by Shasta, who explains it was given to her because she is "Inherent Vice" and could not be insured, but neither Shasta nor Doc know what that meant.