REXX was originally an internal IBM language called REX, for Restructured Extended Executor, replacing the unimaginatively named EXEC and EXEC2 languages on mainframe operating systems. When it was turned into an o-fish-al product, the legal beagles insisted on the name change because of trademark infringement concerns.

REXX was written pretty much singlehandedly by a really bright guy named Mike Cowlishaw at the IBM Hursley (UK) research center.