Origins
The word 'blade' conceals one of English etymology's most striking semantic shifts: a word that originally meant 'leaf' came to mean 'the cutting edge of a weapon.' The transformation happened through a visual metaphor so natural that speakers barely noticed it — a sword's flat metal surface resembles a large, rigid leaf.
Old English 'blæd' meant 'leaf' or 'blade of grass,' and nothing else. It descended from Proto-Germanic *bladą, which meant 'leaf' across all the Germanic languages: German 'Blatt' (leaf, sheet of paper), Dutch 'blad' (leaf), Old Norse 'blað' (leaf), Swedish 'blad,' and Danish 'blad' all preserve the original meaning without the weapon sense. English alone made the metaphorical leap.
The Proto-Germanic form traces back to PIE *bʰleh₃-, a root meaning 'to bloom,' 'to flower,' or 'to flourish.' This root has been extraordinarily productive. Through the Germanic branch, it produced 'bloom' (from Old Norse 'blóm'), 'blossom' (from Old English 'blōstm'), and the archaic sense of 'blow' meaning 'to flower' (as in 'full-blown roses'). Through Latin, the same root gave 'flos' (flower), which generated 'flower' itself (via Old French 'flour'), 'flora,' 'floral,' 'florid,' 'flourish,' and 'Florence' (the 'flowering city'). The connection between leaves, flowers, and blooming all points back to this single ancient concept of vegetable growth.
Old English Period
The weapon sense of 'blade' first appears clearly in the fourteenth century. The metaphor worked because medieval swords typically had broad, flat surfaces that widened from the hilt before tapering to a point — a shape reminiscent of a leaf. The Old English word for the cutting part of a sword had been 'ecg' (edge, surviving in modern 'edge'), and 'blæd' replaced it gradually as the flat-surface metaphor displaced the sharp-edge literal term.
Once established in the weapon context, 'blade' expanded further. By the sixteenth century, it could refer to any flat cutting implement: the blade of a knife, an axe, a razor, a saw, a plough. In the seventeenth century, it extended to the flat rotating arms of a propeller, the flat bone of the shoulder ('shoulder blade'), and the flat part of an oar. In each case, the common thread is flatness — the leaf shape.
A curious slang sense developed in the sixteenth and seventeenth centuries: 'blade' as a term for a dashing, spirited young man — a gallant or a swashbuckler. This usage probably arose from the association with swords: a 'blade' was a man defined by his sword, a fighter. Shakespeare's contemporaries used 'a good blade' to mean 'a brave fellow.' The usage faded by the nineteenth century but survives in the adjective 'debonair,' which sometimes collocated with 'blade' in Restoration comedy.
Scientific Usage
In modern technology, 'blade' continues to extend. Turbine blades, rotor blades, blade servers in computing, and even 'blade runner' (originally a medical smuggler in a 1974 novel, then a film title) all draw on the word's core image of flatness and sharpness.
The word thus offers a complete trajectory from nature to warfare to technology: a Proto-Indo-European root about flowering and growth produced a Germanic word for leaf, which English speakers saw in the shape of a sword, and which engineers later saw in every flat, cutting, or rotating surface. The leaf is still there, hiding in plain sight inside every knife in the kitchen drawer.