Other forms: sharpened, sharpening, sharpens
To sharpen is to make something sharper. Sharpen a knife, sharpen the pencil, sharpen your argument.
A blunt knife is more dangerous than a sharp one, because it slips instead of cutting and needs force behind it. Sharpening removes metal to make a new edge, where honing straightens the edge you have. Note the -en ending that makes verbs from adjectives.
sharpen is a making sharp.
Sharp plus -en. Old English had scearpian; the modern form was rebuilt from the adjective.
Middle English
scharpenen
English, today
sharpen
From Middle English scharpenen, scharpnen, equivalent to sharp + en.
Etymology from Wiktionary, CC BY-SA