Linnaeus tended not to give explanations for the names he gave to species, and there is an opinion ("undoubtably", according to Jobling) that atricilla actually was supposed to be atricapilla and that he misspelled the word accidentally....the so-called “Laughing Gull” (Leucophaeus atricilla)? Incidentally, “atricilla” means “black-tailed”, despite the fact that there is an actual Black-tailed Gull (Larus crassirostris, “thick-billed”), and that the Laughing Gull has a white tail as an adult like most gulls (perhaps there was a mistake and it was supposed to be “atricapilla”?)
I have also once or twice seen it suggested that atricilla is intended as a diminutive of atra rather than meaning "black-tailed". The use of cilla itself to mean "tail" derives from a Medieval mistake.