c. 1600, "single, solitary," from French unique (16c.), from Latin unicus "only, single, sole, alone of its kind," from unus "one" (from PIE root *oi-no- "one, unique"). Meaning "forming the only one of its kind" is attested from 1610s; erroneous sense of "remarkable, uncommon" is attested from mid-19c. Related: Uniquely; uniqueness.
"engage in single combat, fight a duel," 1640s, see duel (n.). Related: Dueled; dueling; duelling.