We all know what level means, right? It's to be even, going neither up nor down. It's the state of being so, like the levels of a house. It's the tool that guarantees it. And all the idioms containing it arise from those. Sure.
But I was quite surprised to learn that the tool is the chronologically first meaning! The English word for such a tool dates from the mid-1300s, taken from the French livel, which comes in turn from the Latin libella -- "scale, balance, unit of weight." The meaning of "horizontally" followed in about 1400, and the line indicated by such a measure in 1530. The phrase on the level originally didn't mean "honest and fair"--it meant "moderate, without great ambition." I had no clue about that one!
The adjective, which I would have assumed to be the oldest definition, didn't in face come along until the early 15th century. Which, granted, is still stinkin' old, LOL. But it's still at least 50 years after the noun, possibly as many as a hundred. The verb followed within another half-century.
Most of the familiar idioms still in use today (level off, level with me...) date from the 1920s. Level off is, not surprisingly, from aviation.