First attested in 明·许仲琳《封神演义》第四十三回: 吾等又不是凡夫俗子，恃强斗勇，皆非仙体 ('We are not ordinary mortals — we don't fight by mere strength, our bodies are immortal'). The phrase contrasts ordinary humans with sages or immortals.
usage
Often self-deprecating ('I'm just an ordinary person, what would I know') or used to contrast someone with the truly elite/extraordinary.
