The story
From site + swap. The term first appears in a Summer 1991 Juggler's World article, "A Notation for Juggling Tricks" by Bruce (Boppo) Tiemann and Bengt Magnusson: : If the juggler is thought of as juggling imaginary sites where the balls could be, as he does these tricks, which ball lands in which site is different from trick to trick. For this reason, we call these tricks "site-swaps." The number of sites is equal to the word length, and may be different from the number of balls actually being juggled.
Etymology from Wiktionary, CC BY-SA 4.0.