mid-14c., "subtle, sophisticated;" late 14c., "eager to know, inquisitive, desirous of seeing" (often in a bad sense), also "wrought with or requiring care and art;" from Old French curios "solicitous, anxious, inquisitive; odd, strange" (Modern French curieux) and directly from Latin curiosus "careful, diligent; inquiring eagerly, meddlesome," akin to cura "care" (see cure (n.)).
The objective sense of "exciting curiosity" is by 1715 in English. In booksellers' catalogues, the word was a euphemism for "erotic, pornographic" (1877); such material was called curiosa (1883), the Latin neuter plural of curiosus. Related: Curiously; curiousness. Curiouser and curiouser is from "Alice's Adventures in Wonderland" (1865).
Curious and inquisitive may be used in a good or a bad sense, but inquisitive is more often, and prying is only, found in the latter. Curious expresses only the desire to know; inquisitive, the effort to find out by inquiry; prying, the effort to find out secrets by looking and working in improper ways. [Century Dictionary]