sow one's wild oats
Behave foolishly, immoderately or promiscuously when young, as in Brad has spent the last couple of years sowing his wild oats, but now he seems ready to settle down . This expression alludes to sowing inferior wild oats instead of good cultivated grain, the verb sowing-that is, "planting seed"-in particular suggesting sexual promiscuity. [Mid-1500s]