| flagitious | adj. | extremely wicked | ||
| flagroot | n. | root of acorus calamus (of the lily family), with supposed healing and hallucinogenic effects, banned in the U.S. for use in food due to its toxicity | ||
| flam | n. | capricious idea or fancy; falsehood, deceit | ||
| flame cell | n. | specialized excretory cell in very simple invertebrates | ||
| flannelette | n. | napped cotton made to resemble flannel | ||
| flapdoodle | n. | nonsense | ||
| flash house | n. | brothel | slang | |
| flash spoon | n. | spoon-shaped metal fishing lure designed to attract through visual action | ||
| flat | n. | fool, dullard | obsolete | |
| flatlings | adv. | with the flat side | ||
| flatus | n. | intestinal gas | ||
| flavescent | adj. | yellowish, turning a pale yellow | ||
| flavid | adj. | yellow, sulphur yellow | ||
| flaw | n. | sudden wind blast; brief gale or storm; abrupt outburst of feeling | ||
| fleer | v. | to grin or laugh in a rude or coarse manner | ||
| flemish | v. | to coil a rope in a pleasing flat spiral pattern | nautical | |
| Flemish horse | n. | extra footrope along yardarm | nautical | |
| flense | v. | to slice away the fat (from a whale or a seal) | ||
| fleuron | n. | flower-shaped decorative element | ||
| flews | n. | hanging flaps of the upper lips of certain dogs | ||
| flex on | v. | to (sometimes subtly) put someone down by showing your superior skills, ability, or possession | slang | |
| flibbertigibbet | n. | gossip; flighty woman | ||
| flinders | n. | small pieces, splinters | ||
| flirt | n. | sudden jerk, quick throw | ||
| flitch | n. | side of bacon; square of whale blubber; halibut steak; lengthwise cut plank of wood | ||
| floccillation | n. | delirious picking at bedclothes, carphology | ||
| floccule | n. | thing resembling a small tuft of wool | ||
| flocculus | n. | dark or light patch on the surface of the sun | ||
| Florida water | n. | Cologne water developed in U.S. in early 19th century, with a distinctive spicy orange scent | ||
| florin | n. | former British coin worth two shillings; gold coin of the city-state of Florence | ||
| flounce | v. | to move in exaggeratedly impatient or angry manner; to make overdramatic movements; to decorate with pleated trimming | ||
| flummery | n. | mere trifling, empty compliment; dish made from oatmeal or flour boiled with water until thick | ||
| flump | v. | to fall or move heavily with a thump | ||
| flytime | n. | season when flies are prevalent and pestering | ||
| foehn | n. | dry warm wind blowing downhill in the Alps, esp. in Switzerland | ||
| foetor | n. | stench, offensive smell (also fetor) | ||
| fog | n. | long and thickly growing grass | ||
| foible | n. | minor quirk or idiosyncrasy; weakness of character; weaker half of sword blade (between middle and the point) | ||
| foin | v. | to thrust or to lunge with a weapon | ||
| foison | n. | plenty, abundance; plentiful harvest; vitality | ||
| foist | v. | to impose an unwanted thing upon, to palm off | ||
| folie à deux | n. | delusion shared by a couple | ||
| foliiferous | adj. | bearing leaves | ||
| folletto | n. | imp, fairy | ||
| foment | v. | to incite, to encourage; to apply heat or ointment to (body part) | ||
| fomes | n. | objects which may carry infectious organisms after contact with contagious individual (pl. fomites) | ||
| fons et origo | n. | source and origin | Latin | |
| fontanel | n. | space between bones of the not fully sutured skull of an infant or fetus | ||
| fontange | n. | high headdress formed of starched cotton and ribbons, worn by upper classes in 17th and 18th Century Europe | ||
| foot up | v. | to total a bill at its bottom |
