But if you really believe that teenage girls (and boys) don’t know what they’re talking about, it’s more likely that they just don’t want you to know what they’re talking about. Teenagers may not be able to drive or vote or stay out past curfew or use the bathroom during school hours without permission, but they can talk. Their speech is the site of rebellion, and their slang provides shelter from adult scrutiny.

Guarding the secret code has become tricky, though. Teenagers used to listen for the telltale click of a parent eavesdropping on the telephone line. Now somebody (or something) is monitoring every keystroke. If an adult picks up a scrap of inscrutable teenager-speak via text or Twitter or a whisper wafting up from the back seat, she can access its definition on Urban Dictionary or Genius (which explains that “ ‘I can’t even’ is a state of speechlessness too deep to even express in any other words”). In 1980, the linguist David Maurer, author of “The Big Con,” a book about underworld slang, wrote that “the migration of words from subculture to dominant culture is sparked by the amount of interaction between these groups,” as well as by the dominant group’s “interest in the behavior patterns” of the other. Parents are perennially nosy about what their teenagers are saying, and nowadays they can just Google it.

Adolescent slang has evolved a clever defense mechanism against the threat of the search engine. Teenagers have always used words to obscure their most sensitive subjects: He’s a total babe, but he sweats this ditz who gets blazed every day after school. Now the most creative linguistic innovations elide the discussion topic entirely. “I can’t even” is a confession interrupted. A close relative of “I can’t even” is the keysmash, a string of actual gibberish — asdf;lkl, maybe — meant to signal that the typist has become so excited that she has lost control of her fingers. Or consider “Your fave could never,” a gleeful taunt meaning roughly “Your favorite [actor] could never [pull off the beach ensemble modeled by my favorite actor, Darren Criss, over which I am literally dying right now].” A reader can only decipher its meaning if she has been briefed on the speaker’s celebrity allegiances and is plugged into the web’s breakneck gossip cycle. The modern revelation doubles as a warning: Reveal less.

This game offers only ephemeral rewards, and that’s part of the point. If teenage slang stayed cool indefinitely, teenagers would never be able to outpace their parents. Maurer identified a few forking paths along which slang might travel. The word can stay slang, recirculating in the counterculture even when new speakers replace the old. It can be “elevated to respectability,” integrated into the discourse of the middle-aged and middle class. Or it can just disappear, a casualty of a cultural shift that renders it moot.