In Hiberno-English people might describe something fun and enjoyable as a gas. For example, “That’s gas”, “A gas laugh”, “Come on, it’ll be gas”, “He’s a gas character”, “Your man is gas” [source].

Last week an Irish friend told me that this expression comes from laughing gas (nitrous oxide), which was used at parties to induce hilarity and euphoria in the guests.

According to The Grammarphobia Blog, the earliest citation in the OED for gas meaning fun was in James Joyce’s 1914 collection of stories, Dubliners, in which one character says he’s brought along a slingshot “to have some gas with the birds.”

According to Historically Speaking, Humphrey Davy noticed that nitrous oxide produced a state of induced euphoria which led to laughter followed by a state of stupor and, finally, a dreamy and sedated state. He introduced it to the British upper class in 1799 and it became used as a recreational drug at “laughing parties”. The term “it’s a gas” soon came to refer to what happened at such parties.