'flat-peaked' originates from Modern English, formed by compounding 'flat' and 'peak' (the adjective 'flat' + past-participle/ adjective form 'peaked').

'flat' goes back to Old Norse 'flatr' (and Proto-Germanic *flatraz) meaning 'broad, level'; 'peak' comes via Middle English from Old English/Old Norse roots referring to a pointed summit. The compound 'flat-peaked' is a modern descriptive formation in English created by combining the two words.

Initially each element meant 'level/broad' ('flat') and 'summit/point' ('peak'); combined in Modern English they describe a summit that is flat rather than pointed, a straightforward literal compound meaning that has been used in descriptive and technical contexts.