To give in under pressure; to admit defeat. The origin of this expression is disputed. One writer claims it comes from the custom of striking the underside of a table with the knuckles when one was defeated in an argument. However, the noun knuckle once meant the end of any bone at the joint where it forms a protuberance, as at the knee, elbow, and finger joints. The verb to knuckle originally meant “to bend down” or “stoop” (probably from the joint’s bending), and by extension “to comply with” or “submit to”—it was so used from the 1700s on. The first appearance in print of knuckle under with this same meaning was in 1882. To knuckle down, on the other hand, while it originally may have been synonymous with knuckle under, today means to apply oneself resolutely to something. This meaning was first recorded in 1864.