E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
He is sowing his wild oatsindulging the buoyant folly of youth; living in youthful dissipation. The idea is that the mind is a field of good oats, but these pranks are wild oats or weeds sown amongst the good seed, choking it for a time, and about to die out and give place to genuine corn. The corresponding French phrase is Jeter ses premiers faux, which reminds us of Ciceros expression, Nondum illi deferbuit adolescentia. (See OATS.)