This prefix properly denotes replacement of an oxygen by a sulfur, e.g.
PhC(=S)NH2, thiobenzamide. See also entries prefixed thio-.
Source:
PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1371 (https://doi.org/10.1351/pac199567081307)