There are a number of owl swear words and curses. They are named here from lowest to highest.
"Wet Pooper!" -- An insult as it refers to seagulls, who are considered the worst wet poopers of all. Also used in potty humor.
"Great Glaux!" -- Not technically considered a swear word. Used to express surprise. Similar to the phrase "Oh God!"
"Great Glaux in Glamoura!" -- an extended version of Great Glaux.
"Frink" -- Mild profanity, used as follows: "...Frinks me off." or "Frink off!". Similar to the phrase "That p*sses me off!"
"Racdrops!" -- Short for Racoon droppings. Racoons are considered an extremely disgusting animal. Used to express great disgust. Similar to the F word.
"Sprink." -- The absolute worst swear word an owl can utter. Considered of the highest profanity. Absolutely forbidden at Ga'Hoole. Dewlap is known to have fainted when Otulissa uttered it in the library, referring to the spronk book. Otulissa also sprinked Dewlap herself after a long Flint mop. Similar to the C word.