When did dyke become a derogatory term for lesbian and why?

Dear Stephen, Maybe the term came about because as the dyke is between earth and rock, a lesbian is somewhat between a masculine and feminine person - a woman with masculine characteristics. Same as a gay man who might be more feminine. Of course, the world was shocked when Rock Hudson was outed because he was as masculine as a man could be.

The origin of the term is obscure, and many theories have been proposed.[2][3] The Oxford English Dictionary (OED) dates the first recorded use of dike, dyke in 1942, in Berrey and Van den Bark's American Thesaurus of Slang.[4] In his review of a short-lived 1930 Broadway play, Robert Benchley says

[the hero]…is confronted with several engineering problems which he solves by mistake. There’s your story. Interlard it with every known crack which has been made along Broadway for the past two years (and several which haven’t, chief among them being: "Did you employ dikes in building the Barge Canal?" "No, we just had a gang of Italians." This I consider top for the evening.) and there you have “So Was Napoleon.”[5]

In the late 20th and early 21st century, the term has been reclaimed by some lesbian groups. Examples in the culture include the comic strip Dykes to Watch Out For and the traditional Dykes on Bikes that lead pride parades.

Matters came to a head when the United States Patent and Trademark Office denied lesbian motorcycle group Dykes on Bikes a trademark for its name, on the grounds that "dyke" was an offensive word. In 2005, after a prolonged court battle involving testimony on the word's changing role in the lesbian community,[8] the Trademark Trial and Appeal Board permitted the group to register its name.[9] Popular culture still uses dyke as an offensive word, however.

Wouldn't it have been easier just to google "etymology dike" on your own?

First recorded English usage was in a 1942 dictionary of American Slang. It may come from a Dutch slang term for the same thing. For sources, google it yourself.

I always thought it was spelled "dike " and lesbian and dike were different. A lesbian is a woman that likes other women but is still very feminine and a dike is a more manly woman who dresses like a dude and acts like a dude and is attracted only to women and is more of the man in the relationship.?

The preferred spelling is "dyke" but "dike" is also correct.