Surly comes from earlier sirly ('haughty, lordly, like a sir'), from sir (from Old French sire, from Latin senior 'older'); the sense shifted from 'arrogant' to 'rude, bad-tempered'. The historical chain includes Old French sire and Latin senior. The quoted older senses, "haughty, lordly, like a sir", "older", "arrogant", and "rude, bad-tempered", supply the semantic anchors for surly. The formation is best read as borrowed: the word surly carries its older form-history into a compact English lexical shape. In present English it is used as a adj, "bad-tempered and unfriendly". English derivatives include surliness and surlily; related English forms include sir. The result is a adj whose etymology preserves both the older form and the later English specialization.