Both pirates and piracy are words you can use in a SUBJECT (Library of Congress) search. To do a subject search, you must either do a keyword search and then modify the search to change the search field to Subject (Library of Congress), or use the Advanced search.
(piracy OR pirates) AND (somal* OR 21st century)
(piracy OR pirates) AND legislation AND NOT online
(piracy OR pirates) AND 18th century AND (sources OR correspondence)
Note: when searching MOBIUS, you must make your "ANDs" and "ORs" all-caps, or the search will fail.