In 1920, Chicago lawyer Sebastian Hinton trademarked the Junglegym (one word).
He modeled his structure after one devised by his mathematician father, who had envisioned children learning about three-dimensional space by climbing to specific x, y, and z coordinates.
Over the decades, the beloved structure has been updated, and the trademark status lost. However, newer designs still honor what the inventor called the "monkey instinct" of kids.
The (two word) term jungle gym first appeared in 1923; the never-trademarked, closely related term monkey bars traces to 1955.