Of a fruit or flower: containing or producing many (more than four) seeds. Of a plant: having such fruits or flowers.
Late 17th century; earliest use found in Philosophical Transactions of the Royal Society of London. From ancient Greek πολύσπερμος abounding in seed (from πολυ- + σπέρμα seed) + -ous; originally via post-classical Latin polyspermus (J. RayHistoria plantarum I. xi. 578).