Tom Dalzell was thrilled last month when he came across a weird new verb: “rickroll.”

Then he went online and saw that “rickrolling”—the Internet prank that involves sending someone a link to the music video for Rick Astley’s “Never Gonna Give You Up”—has been around for four years. That’s an eon in the world of slang, enough time to render a term stale.

For most people, being late to a language trend isn’t a problem. But Mr. Dalzell, a 59-year-old union leader by day and slang expert after hours, is now in the process of updating the New Partridge Dictionary of Slang and Unconventional English. And as informal language evolves faster than ever, Mr. Dalzell is finding it trickier to keep up.

“Yesterday’s cutting-edge is today’s ho-hum,” he says.

The problem: Slang is born when groups outside the mainstream invent their own language—verbal code that can quickly lose its punch once others catch on. That process used to take a while. But now that social media sites like Facebook and Twitter let people post messages for anyone to see, slang gets exposed much more quickly.

Mr. Dalzell isn’t only dealing with a faster velocity of slang creation. He also contends with fake slang, wherein people invent words and post them online. Anyone, for instance, can edit the Web-based Urban Dictionary. “It’s like naming a star after yourself,” sniffs Mr. Dalzell.

Urban Dictionary CEO Aaron Peckham says “Oh, man, I love reading quotes like that.” The site, he says, provides “a lot of provably false information. It’s supposed to be entertaining.”

Keeping volumes current with slang “is a really big challenge,” says Andrea Hartill, who edits Mr. Dalzell at the New Partridge dictionary. So to keep up, Mr. Dalzell resorts to some creative methods. Last month, he hung out at an online forum for blow-up doll enthusiasts long enough to discover “iDollator,” a person with a fetish for dolls.

To learn about the verb “to bling out”—or, to decorate with excessive ornamentation—he studied a forum for acolytes of the singer Taylor Swift. Read more.

See: The Wall Street Journal