Pugnacious comes from the Latin verb pugnare (meaning "to fight"), which in turn comes from the Latin word for "fist," pugnus.  Another Latin word related to pugnus is pugil, meaning "boxer." Pugil is the source of our word pugilist, which means "fighter" and is used especially of professional boxers. 

The earliest known use of the word "pugnacious" in the English language is in 1642, in the writings of Henry More, a philosopher, poet, and theologian. 

Examples of "pugnacious" in a sentence:  
"The adult males are extremely pugnacious and fight fiercely with one another." 
"There's one pugnacious member on the committee who won't agree to anything." 
"And there seemed to be absolutely no one on the flight who was the least bit pugnacious." 
"It is exceedingly fierce and pugnacious, the males especially fighting with each other for possession of the females." 
"It is a clever, agile and powerful dog, extremely pugnacious in disposition." 
"This suggests that the resemblance to the pugnacious individual may be beneficial in protecting the defenseless cuckoo from enemies."

2 hours ago, Bill W said:

Examples of "pugnacious" in a sentence:   
"And there seemed to be absolutely no one on the flight who was the least bit pugnacious." 

Now this is something I expect on every flight I I board.  

3 hours ago, drpaladin said:

Being  pugnacious alone isn't enough to win a fight.

But might provoke one...

Just the facts:

PUGNACIOUS Definition & Meaning - Merriam-Webster

PUGNACIOUS Synonyms: 103 Similar and Opposite Words | Merriam-Webster Thesaurus