Sentence:  But there was also about him an indescribable air which no mechanic could have acquired in the practice of his handicraft however dishonestly exercised: [...] the air of moral <WSD>nihilism</WSD> common to keepers of gambling hells and disorderly houses; [...] 
Meaning of the word marked with <WSD> tags:  The understanding that all endeavors are devoid of objective meaning.