pre-noun adjectival
extraordinary; uncommon; remarkable
Pre-noun adjectival (rentaishi) used to emphasize that something is far beyond the ordinary level. Often paired with nouns like 努力 (effort), 才能 (talent), or 熱意 (enthusiasm).
彼は並々ならぬ努力で難関試験に合格した。
He passed the difficult exam through extraordinary effort.
その選手には並々ならぬ才能がある。
That athlete has uncommon talent.
並大抵 is also a pre-noun adjectival meaning 'ordinary' or 'average', but it is more often used in negative constructions (e.g., 並大抵ではない) to mean 'not ordinary', while 並々ならぬ directly emphasizes extraordinariness.
Derived from the noun 並み (nami, 'average' or 'ordinary') reduplicated for emphasis, combined with the classical negative form ならぬ (naranu, 'is not') of なり (nari). Literally 'not being ordinary-ordinary', i.e., far from ordinary.