This question was asked on Stack Overflow, but closed as off-topic there.  Before it was closed, it received [this answer][source] (lightly edited by me):

[source]://stackoverflow.com/a/6433463/4850040

> I had guessed that "string" was in use by mathematicians long before
> its adoption in programming languages. Turing machines effectively
> operate on strings. Turing may not have used the term, but it is used
> everywhere in automata textbooks, going back decades.
> 
> The earliest reference I could find was a fragment in Google books of
> a 1944 article ["_Recursively enumerable sets of positive integers and
> their decision problems_"][post] by logician Emil Post in Bulletin of the AMS. I think there is little doubt that he is using "string" in the
> conventional sense used in computer science. Page 286 contains:
> 
> > For working
> > purposes, we introduce the letter *b*, and consider "strings" of 1's
> > and *b*'s such as 11*b*1*bb*1. An operation on such strings such as "*b*1*b*P
> > *produces* P1*bb*1" we term a normal operation. This particular normal
> > operation is applicable only to strings starting with *b*1*b*, and the
> > derived string is then obtained from the given string by first
> > removing the initial *b*1*b*, and then tacking on 1*bb*1 at the end. Thus
> > *b*1*bb* becomes *b*1*bb*1.
> >
> [Paul Callahan]


[post]:http://www.ams.org/journals/bull/1944-50-05/S0002-9904-1944-08111-1/S0002-9904-1944-08111-1.pdf
[Paul Callahan]://stackoverflow.com/users/809458