Make sibilant, perhaps

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Pluralize \Plu"ral*ize\, v. t. [imp. & p. p. Pluralized ; p. pr. & vb. n. Pluralizing .] To make plural by using the plural termination; to attribute plurality to; to express in the plural form. To multiply; to make manifold. [R.]

Wiktionary Word definitions in Wiktionary
alt. 1 (context transitive English) To make plural. 2 (context intransitive English) To take a plural; to assume a plural form. 3 (context transitive English) To multiply; to make manifold. 4 (context intransitive UK English) To hold more than one ecclesiastical ...