From the Greek for "inquirer", it's someone who doubts the claims of philosophical knowledge
This claim was based on the standard Greek distinction between 
knowledge ... thereby attaining the state of mind known to Greek philosophers as 
ataraxia ... the title of "skeptic" (from skeptikos, meaning "inquirer") and "doubters"
; (from... [DOC] [TLE] Riverdale Homeopathy - Skepticism vs. Denialism: We are using the ... [PAR] 2. an attitude of doubt or a disposition to incredulity either in general or toward a 
... about some knowledge claims; but philosophical skeptics have doubted the ...