Webster's Collaborative New Word Dictionary - F
false negative: a person or test result that is incorrectly classified as negative (as for the presence of a health condition) because of imperfect testing methods or procedures.
false positive: a person or test result that is incorrectly classified as positive (as for the presence of a health condition) because of imperfect testing methods or procedures.
fantoosh (adj): Fancy, showy, flashy; stylish, sophisticated; fashionable, exotic. Often used disparagingly, implying ostentation or pretentiousness.
farbrengen (n.): a Hasidic gathering, usually with eating, drinking, singing, and discussion of hasidic teachings, held especially on the Sabbath and other festivals.
farkakte (adj.): covered in excrement. Rare.
faux-hawk: a hairstyle resembling a Mohawk in having a central ridge of upright hair but with the sides gathered or slicked upward or back instead of shaved.
flag-off (n.): the moment at which a race, esp. a motor race, is flagged off; the start of a race. Now chiefly Indian English.
fletched (adj.): of an arrow: fitted with feathers or vanes. Frequently with a modifier, as feather fletched, high fletched, etc.
flex (n.): an act of bragging or showing off.
fluffernutter: a sandwich made with peanut butter and marshmallow crème between two slices of white sandwich bread.
folx (n.): isn’t so much a new word as a new way to spell an old word ("folks"). The definition is “folks—used especially to explicitly signal the inclusion of groups commonly marginalized.” Brewster says folx was tough to define because it only exists in written form. “The word folx at first glance is simply a variant spelling of folks.
forehead thermometer (n): A thermometer that is placed on, passed over, or pointed at the forehead to measure a person’s body temperature.
FTW: an abbreviation for "for the win" —used especially to express approval or support. In social media, FTW is often used to acknowledge a clever or funny response to a question or meme.
FWIW: abbreviation or "for what it’s worth".
Foucauldian (adj. and n.): of, relating to, or characteristic of the ideas or works of the French philosopher and historian Michel Foucault.
fourth trimester: the three month period immediately following giving birth in which the mother typically recovers from childbirth and adjusts to caring for her infant; especially : the first three months of an infant's life.
franger (n.): a condom.
free dive: to swim beneath the surface of water especially at considerable depth without a portable breathing device and typically with a face mask and one or two flippers : to engage in free diving
futzing (n.): ineffectual or trifling activity; messing about; tinkering. Chiefly with around.