As a verb, hawk means (1) to sell goods, especially noisily or aggressively on the street, and (2) to clear the throat of phlegm. One definition of hock is to pawn. Because hock and the first sense of hawk both have to do with selling things, the words are easily confused.
The exact origins of this sense of hock are mysterious, though the OED lists it as originally a U.S. slang word,1 and examples in late 19th-century American sources are easily found in historical Google Books searches (see below). Several of these old examples are in publications associated with northeastern U.S. colleges. And it seems that in the late 19th century, watches were the most commonly hocked items.
In the selling-related sense, hawk is a backformation from hawker, an Anglicization of an old German word.2 In the sense relating to clearing the throat, hawk is probably imitative: it resembles the sound of throat-clearing. Both these senses of hawk are many centuries old.
But it is not all unalloyed joy, for the student having girded up his loins and “hocked” his watch hies him forth to the game and betteth his all. [Harvard Lampoon (1887)]
I’ve got to hock my watch to go, for I must be provided with care fare, at least. [Story in The Puritan (1899)]
Well, he didn’t have any money for his friend, being in bad with his old man, so he hocked his own jewelry and put the money in the in this smuggling scheme. [God’s man: a novel, by George Bronson-Howard (1915)]
Ralph has hocked his bowling ball in order to purchase a suitable gift for Alice; she, in turn, gives him a bowling bag. [New York Times (1987)]
Even when he runs out of liquor and stumbles interminably down New York’s Bowery trying to hock his typewriter on Yom Kippur day (The Jewish Holy Day of fast), the camera follows him relentlessly. [BBC (2010)]
She wore a ”demure” dress and hawked her perfume on the red carpet. [Brisbane Times]
She was seen making out with her newly adopted dog and then hawking a loogie over the balcony. [The Blemish]
At that moment, a stranger approached the group hawking a gold watch in one hand, and in the other, a fishing rod. [Reuters]
O.K. all you fanboys hawking up joy balls over The Incredible Hulk this weekend, there’s something coming for fangirls, too. [Film School Rejects]