What is the most esoteric programming language under the programming languages that were not intended to be esoteric? The first language that comes to my mind is APL. Any body having a better suggestion?
SNOBOL, an early string-based pattern-matching language
K, a database query/programming language
PROSE, a dynamic systems modeling language from the 1970s that introduced automatic differentiation
Among languages most people would have heard of, though, my vote for the most esoteric goes to Prolog.
[0] https://matt.might.net/articles/c++-template-meta-programmin...
It's actually a metaprogrammable functional programming language. It's possible to do a ridiculous amount of stuff in it. For example, I managed to write a pure GNU Make implementation of the find command. I tried to implement a small version autoconf as well but stopped because I could no longer understand what was going on.
sendmail.cf configuration file.
text munging languages from the Unix environment, like ed and sed.
text munging languages from elsewhere like Teco.
nroff/troff.
Interactive Vi/Vim commands look like an esolang when you record them into a register and confront what that looks like.