skeptic (n.) Look up skeptic at Dictionary.com
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" (see scope (n.1)).
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.
skepticism (n.) Look up skepticism at Dictionary.com
also scepticism, 1640s, from skeptic + -ism. Specifically regarding Christian religion, from 1800.
skeptical (adj.) Look up skeptical at Dictionary.com
also sceptical, 1630s; see skeptic + -al (1). Related: Skeptically.
Pyrrhonic (adj.) Look up Pyrrhonic at Dictionary.com
1590s, "pertaining to Pyrrho," skeptic philosopher of Elis (c.360-c.275 B.C.E.), who held the impossibility of attaining certainty of knowledge. Related: Pyrrhonism; Pyrrhonist.
sceptic (n.) Look up sceptic at Dictionary.com
chiefly British English spelling of skeptic (q.v.). Related: Sceptical; sceptically; scepticism.
philosophe (n.) Look up philosophe at Dictionary.com
"Enlightenment rationalist and skeptic," especially in reference to any of the French Encyclopædists, often disparaging (when used by believers), 1774, from French philosophe, literally "philosopher" (see philosopher). Usually italicized in English, but nativized by Peter Gay ("The Enlightenment," 1966) and others. Also philosophist (1798).