Here is an extract from the impact of lambda calculus in logic and computer science.
In Russel and Whitehead's [1910-13] Principia Mathematica the notation for function f with f(x) = 2x + 1 is 2x̂ + 1. Church originally intended to use the notation x̂.2x + 1. The typesetter could not position the hat on the of the x and placed it in front of it, resulting in ^x.2x + 1. Then another typesetter changed it into λx.2x + 1.
Kwang Yul Seo 
I found an interesting behind story about the origin of notation λ in this video.
“Why in the world, did you choose lambda as your operator?”
Church’s response: “eeny meeny miny moe”.
around 9:45