A remarkable etymology has been brought to my attention by the indefatigable aldiboronti at Wordorigins.org: "the word surly is
no more than an alteration of sirly, which meant lordly, haughty, imperious,
acting like a sir in fact." A couple of citations for the original form:1579
SPENSER Sheph. Cal. July 203 Sike syrlye shepheards han we none, They keepen all the path. 1600 HOLLAND Livy XXXV. xxxviii. 911 Syrly lords (say they) were the
Macedonians, and rigorous.