From over + -most. <span class="searchmatch">overmost</span> (not comparable) (obsolete) The highest]] (in authority, status, or elevation) 1900, F. S. Ellis, The Golden Legend, translation...
northwesternmost northwestmost oldermost oralmost outermost outmost outwardmost <span class="searchmatch">overmost</span> periostealmost posteriormost proximalmost radialmost rearmost rearwardmost...