means A stud is an upright support post in a wall frame, a small projecting knob or peg used for fastening or decoration, or—informally—a man admired for his sexual prowess or general impressiveness.

from From Old English 'studu' meaning a post, pillar, or upright support — the literal timber holding up a wall. The metal-knob sense ('studded with jewels') grew from the same idea of something fixed upright and protruding. The 'virile man' sense is a 19th-century slide from 'stud' meaning a male horse kept for breeding (as in 'stud farm'), which itself traces back to Old English 'stod', a herd of horses — a cousin of words across the Germanic family for 'standing' and 'stallion-keeping'.