Turn Over a New Leaf

In this phrase, a "leaf" isn't a plant leaf, it's an old and still-surviving word for a page in a book (hence "loose-leaf paper"). Turning over a new leaf meant physically flipping to a fresh, blank page, an obvious metaphor for putting a messy or unwanted chapter behind you and starting again cleanly. The phrase in something close to its current form is recorded from the mid-1500s, in John Heywood's 1546 collection of English proverbs, with the specifically "new" wording following soon after in Roger Ascham's 1570 book The Scholemaster.