A noun is a person, place, or thing. A verb is an action word. An adjective is a word that modifies a noun.
Except when they aren't.
English is a slippery, supple tongue, hard to corner and classify. I just learned a term for this state of flux: anthimeria (an-thi-MER-i-a), the use of a word outside of its customary part of speech--most commonly, but not exclusively, the use of a noun as a verb. Ben Yagoda, subbing for William Safir in the "On Language" column of the Sunday New York Times Magazine, offers a lively introduction to the concept, which he says "gives English an invigorating slap upside the head." ("Upside": how many anthimeric changes can you ring on that word?)
English speakers and writers have been turning nouns into verbs, and vice versa, for centuries, and we are the richer for it. Yagoda points out that Shakespeare was the first to use "season" and "dog" as verbs and "design" and "scuffle" as nouns. More recently, I would count "to eyeball" and "to blacklist" as nervy verb-y recastings of nouns. In street English, anthimeria runs rampant and even amok, as in "my bad," "dumb down," "weird out," and "pimp my ride," which Yagoda calls doubly anthimeric. ("Ride" came into Old English as a verb, was first recorded as a noun in 1759, and acquired the meaning "a motor vehicle" as early as 1930.) Or consider the noun-adjectives (nounjects?) in "bitch slap" and "that's dope."
Anthimeria isn't always so colorful or evocative. In its more graceless forms,anthimeria is the plague of corporate English, which piles up Latin-derived noun-verbs (nerbs?) in the misguided belief that they sound hifalutin: "Impact the bottom line." "Leverage our core competencies." "Access those files." It's not anthimeria's fault, poor girl; these constructions are culpable on grounds of ugliness, triteness, and aridity. Unlike "eyeball" and "blacklist," which have very old English origins and concrete meanings, "impact," "leverage," et al., suck the air out of every sentence they inhabit.
And don't even get me started on "incent," to mean "provide an incentive." Incensed, I am.
Meanwhile, corporate lawyers rack up billable hours primly telling malefactors to stop saying "Please xerox that document" or "I googled that hottie." Go figure: Companies spend millions turning their brands into household words--words that take on vigorous independent lives, stretch their muscles, and through the magic of anthimeria become active parts of speech--and then spend millions more to keep us from using the words in our households. C'mon, guys: chill.