When you impugn, you hazard repugnant pugnacity. More simply put, you risk insulting someone so greatly that they may punch you in response. The belligerent implications of impugn are to be expected in a word that derives from the Latin verb pugnare, which means "to fight." In its earliest known English uses in the 1300s, impugn could refer to a physical attack (as in, "the troops impugned the city") as well as to figurative assaults involving verbal contradiction or dispute. Over time, though, the sense of physical battling has become obsolete and the "calling into question" sense has predominated. As you might expect, pugnare also gave English other fighting words, including repugnant and pugnacity.