One meaning of the word lustrum is a sacrifice for purification done every five years; another is a house of ill repute. I'd always figured that the two were complete homophones.
However, someone told me recently that the sacrifice has a long ū in its stem, while the debauchery has a short ŭ, which would be a convenient way to tell them apart.
Is this true? How can I check? My usual reference source is L&S, which unfortunately doesn't mark vowel quantity in closed syllables (…or even consistently in open ones); Wiktionary does show different vowel lengths, but I'm not sure where it gets that information from or how reliable it is.