One is the first positive odd number, an integer not evenly divisible by two. Odd, from the Old Norse oddi, point of land, triangle, the odd point sticking out, not lining up. As Dr. Math explains, the words we use are pictures of the shapes a number makes.
In English, the numeral 1 resembles the letter I. The picture I makes is the shape of first person, singular. When spoken, the shape of the mouth draws apart, one’s two lips separating in what looks to be either the start of a smile or one’s initial pain.