An archaic or obsolete spelling of 'fewness'; the quality or state of being few in number.
From Old English 'feaw' (few) plus '-nes' (archaic spelling of '-ness'). Before standardized spelling in the 16th century, English had multiple acceptable spellings for the same word, and '-nes' was an older variant of '-ness' that gradually fell out of use.
Words like 'fewnes' show that modern spelling standardization is actually recent—before the 1500s, people spelled words however they wanted, and 'fewnes' was just as correct as 'fewness'! This explains why Shakespeare's spelling looks so strange to us.
Complete word intelligence in one call. Free tier — 50 lookups/day.