The description you found was of American portraitist, Gilbert Stuart:
For the impulsive, unreliable Stuart, who left a trail of incomplete paintings and irate clients in his wake, George Washington emerged as the savior who would rescue him from insistent creditors. "When I can net a sum sufficient to take me to America, I shall be off my native soil," he confided eagerly to a friend. "There I expect to make a fortune by Washington alone. I calculate upon making a plurality of his portraits. . . and if I should be fortunate, I will repay my English and Irish creditors." In a self-portrait daubed years earlier, Stuart presented himself as a restless soul, with tousled reddish-brown hair, keen blue eyes, a strongly marked nose, and a pugnacious chin. This harried, disheveled man was scarcely the sort to appeal to the immaculately formal George Washington.
Here's the self-portrait referred to in the passage (hardly a pugnacious chin, I think):
Here's a later portrait of him by Charles Willson Peale (not sure how his nose line changed so much, but the chin's a bit more pugnacious)
Ah, here we go. Pugnacity!
By the way, describing chins as pugnacious is much more common than I thought. And pugnacious jaw is even more common.
One piece I think is missing from the other answers is why pugnacious might mean jutting or square-jawed. I found this at Etymonline:
1640s, from L. pugnacis, gen. of pugnax "combative," from pugnare "to fight," from pugnus "fist" . . .
So, I posit that a pugnacious chin is square because it is shaped like a fist.