-ii

The story

From rebracketing of plural forms like radii (plural of radius), borrowed from Latin. Latin forms the nominative plural of second-declension nouns by replacing -us with -ī, which results in plural forms that end in -iī for nouns with singulars ending in -ius. Some English speakers occasionally extend -ii to form nonstandard plural forms of English nouns that end simply with the letters -us or with other letter sequences with a similar pronunciation (such as -is /ɪs/).