Glue-sniffing is the habit of breathing in the strong gases that come off glue, on purpose, because those gases can make the head spin and give a short, strange high. It is not about sticking paper. It is a cheap kind of solvent abuse: the person is using the glue as a drug. You meet the word in news, in warnings to young people, and in older writing about street life. It has a heavy, slightly dated sound, like a headline from decades ago, though the habit itself has not gone away. Everyday talk may just say sniffing glue; the hyphenated word names the whole habit. It is dangerous. The gases can harm the brain, the heart, and the lungs, and people have died from it. Though actually, the “pleasure” is brief and ugly — dizziness, a thick head, then often sickness — not a soft, happy feeling.