Lacking belief, faith, or conviction; skeptical or incredulous.
From belief + -less (suffix meaning 'without' or 'lacking'). The opposite construction of 'beliefful,' showing how the -less suffix inverts meaning.
English is beautifully symmetrical: you can be 'beliefful' or 'beliefless,' 'hopeful' or 'hopeless'—the -less suffix is like hitting the opposite switch! Medieval people would find modern skepticism 'beliefless' in every sense.
Complete word intelligence in one call. Free tier — 50 lookups/day.