What is the origin of λ for empty string? I usually use the symbol $\varepsilon$ for empty string (empty word or null string). But I know some people use $\lambda$ instead of $\varepsilon$.

I think $\varepsilon$ is derived from the word "Empty". However I don't know what's the origin of $\lambda$.

In automata theory, there is the epsilon transition of automata, and it's also said to be the lambda transition. For example, JFLAP software uses $\lambda$ for the label of epsilon transitions by default.

I googled on the origin and searched cs.stackexchange, but I couldn't find. Does anyone know a reference that describes this?
The German Wikipedia claims that $\lambda$ comes from "leer", which means "empty" in German. That seems plausible, as German used to be one of the major languages in mathematics.

Chomsky used $I$ as the empty string (or actually as the identity element for string concatenation) in his early papers. Some people in combinatorics still use $1$ as the empty string, with the same justification.