The problem with such a lexicon would be that there have been a lot of slang, dialect, idioms, and unofficial words that 99% of people have never heard of. Besides that, the English language is a very old language. Old English is quite similar to old Danish, for instance. Such a list would be over 99% words that most people could not use or remember effectively.

I know there are courses and programs to improve vocabulary which are good for anyone, even those who already have a pretty good vocabulary. Hopefully, someone will recommend some. I am unable to do this at the moment.

I don’t know about a particular website, but there is a short TV series that goes over slang in America that my husband and I both enjoyed. He wasn’t raised in America so slang and sayings are tricky for him, and I found it interesting to know how our sayings developed over time.

Here’s the link to watch the shows online: https://play.history.com/shows/americas-secret-slang/season-1/episode-1

If you want to improve your vocabulary read a book with a dictionary by your side so you can learn new words as you go along. Kindle books have useful dictionaries built in that you might want to try.

I wouldn’t expect the moon. No dictionary or website contains everything. There are hundreds of dictionaries for different purposes; etymological, technical, dialect, slang and none is comprehensive.

@Jleslie That link is from A and E. It requires that you sign into or sign up for television service (cable, etc.). The person who asked this question is most likely not from a country that carries A and E.

Here are some sources for learning English idioms.

An idiom dictionary: https://www.theidioms.com/

This one is an amusing explanation behind some British idioms. https://globalgraduates.com/articles/20-bizarre-english-idioms-and-how-to-explain-them

These appear to be American English idioms: https://myenglishroutine.com/funny-english-idioms/

In additon to idioms, you mention you want to expand your vocabulary. Here’s a vocabulary list for grade school children. I think there may be a few words you don’t recognize. Remember also that many words have several meanings, and some have different spellings but the same pronunciation. https://www.greatschools.org/gk/articles/vocabulary-words-for-1st-through-12th-graders/

After you work through those, you might want to look up an English vocabulary list for college.

@snowberry You don’t sign into A&E, you sign into a provider. They might have Hulu or one of the others, or a friend who has one of the others. Or, someone else on the Q might be interested here in America.

Yes. OED.com

For the low, low price of $295 a year.

(Unless you’re living in the UK in which case go and join your local library as it will give you free on line access to the OED and literally 1000’s of other similar sites. They should probably advertise this more)

The OED online might well be accessible in the Us through public library databases. Check your library’s web site.

The Oxford English Dictionary (OED) is the closest you’ll come to a comprehensive inventory. It is also still available (isn’t it?) in printed book form. I have a two-volume edition (four three-column pages per page; very small print) and a one-volume edition (nine pages per page; very, very small print). They come with magnifying glasses. I think they’re beautiful.

But you can do well with your vocabulary if you just keep reading, reading various authors and reading in a range of subject matter. Read periodicals as well as books. Look up any word you don’t know; but first try to guess their meaning from context clues and see how close you come. You might even start a notebook of words you’re learning.

A good command of English is much more than vocabulary, though. And good models of grammar, composition, structure, and style and harder and harder to come by. Even venerable publications such as the Washington Post and Harper’s (magazine, not Bazaar) are edited with less care than they used to be.

Read some older books, even if the language is a little unfamiliar. They used to be more competently edited and proofread than most of them are now.

P.S. You are smart to recognize that speaking and writing well are aspects of perceived intelligence. A person who doesn’t have those skills might still be bright and even brilliant, but it’s generally harder to recognize. Spoken and written English are often your ambassadors, preceding you in applications, resumes, phone calls, texts, profiles, and other media. Your first impressions get there before you do.

The full OED comes in 20 volumes and currently costs about £900 however the last time it was updated in printed form was 1989. They have published a supplements and additions every so often with new words and amendments etc but in December 2020 they added or altered 500 words. The online version is much better.