Any one of several prickly composite plants, especially those of the genera Cnicus, Craduus, and Onopordon. The name is often also applied to other prickly plants. [1913 Webster]

thistle, n.
1 any of various prickly composite herbaceous plants of the genus Cirsium, Carlina, or Carduus etc., usu. with globular heads of purple flowers.
2 this as the Scottish national emblem.

Etymology:
OE thistel f. Gmc

N  sharpness, acuity, acumination, spinosity, point, spike, spine, spicule, spiculum, needle, hypodermic needle, tack, nail, pin, prick, prickle, spur, rowel, barb, spit, cusp, horn, antler, snag, tag thorn, bristle, Adam's needle, bear grass, tine, yucca, nib, tooth, tusk, spoke, cog, ratchet, crag, crest, arete, cone peak, sugar loaf, pike, aiguille, spire, pyramid, steeple, beard, chevaux de frise, porcupine, hedgehog, brier, bramble, thistle, comb, awn, beggar's lice, bur, burr, catchweed, cleavers, clivers, goose, grass, hairif, hariff, flax comb, hackle, hatchel, heckle, wedge, knife edge, cutting edge, blade, edge tool, cutlery, knife, penknife, whittle, razor, razor blade, safety razor, straight razor, electric razor, scalpel, bistoury, lancet, plowshare, coulter, colter, hatchet, ax, pickax, mattock, pick, adze, gill, billhook, cleaver, cutter, scythe, sickle, scissors, shears, pruning shears, cutters, wire cutters, nail clipper, paper cutter, sword, bodkin, belduque, bowie knife, paring knife, bushwhacker, drawing knife, drawing shave, microtome, chisel, screwdriver blade, flint blade, guillotine, sharpener, hone, strop, grindstone, whetstone, novaculite, steel, emery, sharp, keen, acute, acicular, aciform, aculeated, acuminated, pointed, tapering, conical, pyramidal, mucronate, mucronated, spindle shaped, needle shaped, spiked, spiky, ensiform, peaked, salient, cusped, cuspidate, cuspidated, cornute, cornuted, cornicultate, prickly, spiny, spinous, spicular, thorny, bristling, muricated, pectinated, studded, thistly, briary, craggy, snaggy, digitated, two-edged, fusiform, dentiform, denticulated, toothed, odontoid, starlike, stellated, stelliform, sagittate, sagittiform, arrowheaded, arrowy, barbed, spurred, acinaciform, apiculate, apiculated, aristate, awned, awny, bearded, calamiform, cone-shaped, coniform, crestate, echinate, gladiate, lanceolate, lanciform, awl, awl-shaped, lance-shaped, awl- shaped, scimitar-shaped, sword-shaped, setarious, spinuliferous, subulate, tetrahedral, xiphoid, cutting, sharp edged, knife edged, sharp as a razor, keen as a razor, sharp as a needle, sharp as a tack, sharpened, set.

N  levity, lightness, imponderability, buoyancy, volatility, feather, dust, mote, down, thistle, down, flue, cobweb, gossamer, straw, cork, bubble, balloon, float, buoy, ether, air, leaven, ferment, barm, yeast, lighter-than-air balloon, helium balloon, hydrogen balloon, hot air balloon, convection, thermal draft, thermal, light, subtile, airy, imponderous, imponderable, astatic, weightless, ethereal, sublimated, gossamery, suberose, suberous, uncompressed, volatile, buoyant, floating, portable, light as a feather, light as a thistle, light as air, lighter than air, rise like a balloon, float like a balloon.

barnaby's thistle, blessed thistle, blue thistle, boar thistle, brook thistle, bull thistle, canada thistle, carline thistle, common carline thistle, cotton thistle, creeping thistle, european woolly thistle, field thistle, globe thistle, golden thistle, holy thistle, horse thistle, lady's thistle, melancholy thistle, milk thistle, musk thistle, nodding thistle, our lady's mild thistle, plume thistle, plumed thistle, russian thistle, scotch thistle, sow thistle, spear thistle, stemless carline thistle, welted thistle, white thistle, woolly thistle