Definition of lusus:
(n) :
(mythology) The supposed son or companion of Bacchus, god of wine and divine madness, to whom Portuguese national mythology attributed the foundation of ancient Lusitania and the fatherhood of the Lusitanians.
(n) :
Ellipsis of lusus naturae. [(historical, originally) A sportive quality in Nature to which abnormal forms were formerly ascribed; a supposed capricious act of Nature regarded as the cause and origin of anomalies in a given organism.]