What is the meaning of the pejorative form of “gay”?

Dictionaries don’t define the pejorative use of gay, but the term is used in common parlance. For example:

That’s so gay.

or

You’re gay.

Is there a way of establishing what gay means when used pejoratively?

Dictionaries don't define the pejorative use of gay

Some of them do:

• Dictionary.com: Gay: "awkward, stupid, or bad; lame: This game is really gay.
• Oxford Dictionaries: Gay: "Foolish, stupid, or unimpressive: he thinks the obsession with celebrity is totally gay."
• Wiktionary: Gay (English): "Used to express dislike: lame, uncool, stupid: This game is gay; let’s play a different one. = I dislike this game; let’s play a different one."

Anecdotally, growing up in the nineteen-eighties and nineties (in England and then the US), I perceived "gay" as an insult growing in popularity directly alongside the widening awareness of the concept of "gay" as an identity. I was acutely sensitive to the one due to my private identification with the other, and I am quite certain none of the kids I knew were using some hypothetical other tradition of the word.

They called things "gay" precisely because gay people were different and shameful, and because adults discouraged mention of the subject, putting the word into the category of "naughty" (and therefore enjoyable) words that covers sex, genitalia and bodily functions. Using the word in this way reinforced the negative connotations attached to gay people, and continues to do so even as diverse sexual identities become gradually more accepted.

Looking for separate explanations of the derogatory and descriptive senses of the word is misdirected; the referent is the same, it's just that the derogatory uses are attempts to hang disapproving connotations upon it.

UPDATE: here's a snapshot of the evolving popular meaning of "gay" in 1986.

No, gay used pejoratively has no special meaning to distinguish it from any other slight meant to disparage and discredit those whom it refers to. Their only real “meaning” is to be mean. It’s just like calling something lame, which is casually insensitive to people who cannot walk or run easily.

In this way, this is just like any other slur based on race or national origin or religion or sex or disability physical or mental, or on anything else that the execrable dregs of humanity use against their fellow man.

I will not besmirch this site by enumerating a list of spiteful examples of these, but I’m sure countless many of the same bilious character come readily to mind once you try to think of some.

Such insults are nothing but base stigmatizations — rank put-downs, if you would — committed by xenophobic churls and others of equally low breeding, empathy, intellect, courtesy, and judgement.

Whether done in casual passing or as an active affront, every single one of these slurs is verbal violence but thinly veiled and summarily vile. They can be used as hate speech — and often are. Their intent is to dehumanize and sometimes even demonize those whom such speakers perceive to be “different” from themselves, and by extension inherently inferior from them.

That is their “meaning”.