According to [PDF] LISP: Program is Data - A Historical Perspective on MACLISP published in Proceedings of the 1977 MACSYMA Users' Conference:
In 1973, White coded an s-expression hashing algorithm called SXHASH, which has been useful to routines doing canonicalization of list structure [...]
White here refers to Jon L White I guess.
"SX" is a common abbreviation for "S-Expression" (Structured Expression) frequently used in the Incompatible Timesharing System source code.