| a calculus or concretion found in the stomach or intestines of certain animals, esp. ruminants, formerly reputed to be an effective remedy for poison. |
| a stew of meat, vegetables, potatoes, etc. |
"That wilfull and vnruly age, which lacketh rypenes and discretion, and (as wee saye) hath not sowed all theyr wyeld Oates." [Thomas Newton, "Lemnie's Touchstone of complexions," 1576]Hence, to feel (one's) oats "be lively," 1831, originally Amer.Eng.
feel one's oats
Feel frisky or animated, as in School was out, and they were feeling their oats. This usage alludes to the behavior of a horse after having been fed. [Early 1800s]
Display self-importance, as in He was feeling his oats, bossing everyone around. [Mid-1800s]