skeptic (n.)

also sceptic, 1580s, "member of an ancient Greek school that doubted the possibility of real knowledge," from Middle French sceptique and directly from Latin scepticus "the sect of the Skeptics," from Greek skeptikos (plural Skeptikoi "the Skeptics, followers of Pyrrho"), noun use of adjective meaning "inquiring, reflective" (the name taken by the disciples of the Greek philosopher Pyrrho, who lived c. 360-c. 270 B.C.E.), related to skeptesthai "to reflect, look, view" (from PIE root *spek- "to observe").
Skeptic does not mean him who doubts, but him who investigates or researches as opposed to him who asserts and thinks that he has found. [Miguel de Unamuno, "Essays and Soliloquies," 1924]
The extended sense of "one with a doubting attitude" first recorded 1610s. The sk- spelling is an early 17c. Greek revival and is preferred in U.S. As a verb, scepticize (1690s) failed to catch on.