Sentence:  In the South every negro preacher is "ex officio" a D.D., and is commonly addressed as "Doctor". This enables white Southerners to show a decent respect for his sacred office, and yet avoid the <WSD>solecism</WSD> of calling him "Mister". 
Meaning of the word marked with <WSD> tags:  A faux pas or breach of etiquette; a transgression against the norms of expected behavior.