Sharpen comes from sharp plus the verb-forming suffix -en; sharp is from Old English scearp ('having a cutting edge'), from Proto-Germanic *skarpaz, from Proto-Indo-European *(s)ker- ('to cut'). The historical chain includes Old English scearp and Proto-Germanic *skarpaz. The quoted older senses, "having a cutting edge" and "to cut", supply the semantic anchors for sharpen. The formation is best read as suffixal and borrowed: the word sharpen carries its older form-history into a compact English lexical shape. In present English it is used as a verb, "to make or become sharper, keener, or more acute". English derivatives include sharpener and sharpening; related English forms include sharp and sharpness. The result is a verb whose etymology preserves both the older form and the later English specialization.