Sowing Your Wild Oats
Wild oats, or the "crop that one will regret sowing," was first used in the 1560s, in reference to the folly of sowing these instead of good grain. Protestant clergyman Thomas Becon is attributed to have said a variation of this phrase first.
This exact phrasing was supposedly first said in 1952, and the expression means to pursue a useless endeavor. Farmers have long hated wild oats because they are a useless crop and hard to separate from cultivated oats.
Also, there is a sexual connotation attached to the idiom that implies that a young man is spreading his seeds without purpose. If a man sows his wild oats, he has a period of his life when he does a lot of exciting things and engages in non-monogamous, often elicit sexual relationships.