A discourtesy is an act that fails in ordinary politeness. A small discourtesy, meant no discourtesy, treated with discourtesy.
It is formal and understated, and understatement is the point — calling something a discourtesy in a complaint is heavier than calling it rude. Note courtesy beneath it, from court, since courtly manners were the standard.
1noundiscourtesyAn act that fails in ordinary politeness.
“Arriving late without an apology was a discourtesy to everyone who had waited.”
discourtesy is a rudeness.
Old French
curteisie
Anglo-Norman
curtesie
Middle English
curteisie
English
courtesy
English, today
discourtesy
From dis- + courtesy. courtesy comes from Middle English curteisie, curtesie, from Anglo-Norman curtesie, from Old French curteisie, cortoisie.[http://books.google.ca/books?id=aDhGlKL3h00C&pg=PA97&lpg=PA97&dq=courtesy+etymology&source=bl&ots=t3EKf41eNo&sig=orGVGkmIiBWn0P8md
Etymology from Wiktionary, CC BY-SA