"It's a Gas"

Am I remembering the dialogue correctly, that during Eva's trial, she sardonically told Kevin, when he asked how it (the trial) was going, "it's a gas"?

I meant to look the slang phrase up, and just having finished the series, recalled that intention.

At first I assumed it was quite an anachronism, as I associated it with 1960's slang. Then I thought, "maybe it entered into slang from the use of nitrous oxide".

However, the earliest citing I can find for the phrase as used in "Copper", dates to *possibly* 1914, but more certainly 1957.

I know historical veracity wasn't the strong suit here, but the phrase seemed too deliberate to be a mere slip.

Just wondering what any other viewers made of that anachronistic line of dialogue.