'frontmost' is formed from 'front' (from Old French 'front', ultimately from Latin 'frons, frondis', meaning 'forehead, front') combined with the superlative 'most' (from Old English 'mæst'/'māst').

'front' entered Middle English from Old French 'front' (from Latin 'frons'), and the English superlative element 'most' derives from Old English; the compounded adjective 'frontmost' appeared in Modern English by combining these elements to indicate 'most at the front'.

Originally 'front' was associated with the physical 'forehead' or 'fore part'; over time the sense shifted to 'foremost position', and 'frontmost' came to mean 'nearest the front' or 'most prominent at the front'.