(n.) violation of what is held sacred
It’s tempting to presume that hiding somewhere inside sacrilege is the word religion, or else that somewhere in its history, a word like sacrilegious must at least cross paths with a word like religious. It’s tempting, alright, but misguided—any similarity between the two is actually sheer coincidence.
Sacrilege brings together two Latin word roots. The first, sacrum, essentially means “shrine”, “sanctuary”, or more loosely “sacred thing”, and is a derivative of the Latin word for “sacred”, sacer. That’s also where words like sacrifice, sacrosanct and sacrament come from, as well as a clutch of less familiar terms like sacerdotal (an adjective meaning “pertaining to priests”), sacellum (a small, roofless temple), and the word sacrum itself (used in English as the name of the bone in the pelvis, supposedly so-called because of its ancient importance in animal sacrifices).
The second half of sacrilege is not from the same root as religion, but rather comes from the Latin verb legere. It was variously used to mean “to take or to choose”, “to gather together”, “to sift through”, “to remove or extract”, “to pick fruit from a tree”, and “to purloin, or take without permission” in Latin, and all those different shades of meaning survive in an array of derivative words still in use today.
A quick glance through a dictionary at all those words tracing some kind of etymological connection to legere will throw up as diverse a group as collect, neglect (literally “to not pick”), legume, and elegant (which originally described someone with select, dainty tastes), alongside much less familiar examples like delectus (“a choice selection of literary passages”), sortilege (a sorcerer or soothsayer, literally “one who chooses lots”) and ablectic (a seventeenth-century adjective meaning “set aside to be sold”).
Put those two roots back together then, and you’ll have sacrilege—a word that literally means “the stealing of sacred things”.