The word 'steepen' is a transparent modern English formation, created by applying the native Germanic suffix '-en' to the existing adjective 'steep'. The adjective 'steep' itself originates from the Old English stēap, which carried meanings of height, loftiness, and depth, and is related to other Germanic words like Old Frisian stāp and Middle Dutch stoop. The suffix '-en' is a productive morpheme in English, historically used to create verbs that mean "to become or cause to be" the quality denoted by the root adjective, as seen in pairs like 'deep/deepen', 'wide/widen', and 'sharp/sharpen'. Thus, the logic of 'steepen' is direct: it means to take on or impart the characteristic of being steep, describing a change in gradient or slope.

Structure: steep (adjective) + en (verb-forming suffix) - steep: From Old English stēap (high, lofty, deep, prominent), of Germanic origin. - en: A verb-forming suffix from Old English -nian, used to form verbs from adjectives or nouns, meaning "to become or make something have the quality of".