sceptic an ancient or modern philosopher who denies the possibility of knowledge, or even rational belief, in some sphere. The leading ancient sceptic was Pyrrho, whose followers at the Academy vigorously opposed Stoicism. Modern sceptics have held diverse views: the most extreme have doubted whether any knowledge at all of the external world is possible, while others have questioned the existence of objects beyond our experience of them.

The term comes (in the late 16th century) via French or Latin from Greek skeptikos, from skepsis ‘inquiry, doubt’.

sceptic, U.S. skeptic one who doubts. XVI. — F. sceptique or L. scepticus, in sb. pl. scepticī followers of the Greek philosopher Pyrrho of Elis — Gr. skeptikǒs, sb. pl. skeptikoí, f. sképtesthai look about, consider, observe. The pronunc. has been infl. by Gr.
Hence sceptical, scepticism XVII.