1)  Characterized by ludicrous or repulsive distortion; ugly or outlandish  in appearance.

Leonardo grotesque headsLeonardo da Vinci. Studies of grotesque heads. Ca. 1490.

2)  A decorative painting style invented in ancient Rome–and rediscovered in Renaissance Europe–in which foliage and imaginary human and animal figures are combined in fanciful, extravagant designs.

Grotesque Golden HouseGrotesque

Left: Grotesque ceiling decoration in fresco. Nero’s Golden House, Rome. 64-68 CE.  Right: Nicoletto da Modena. Design for a Panel of Grotesque Ornament. Engraving on paper. Second quarter of the 16th century. Victoria and Albert Museum, London.