In as much as the Oxford English Dictionary defines it as "to fluster," it's generally received as a portmanteau of "fluster" and "frustrate." English is chock-full of portmanteaus: spork (spoon+fork), biopic (biographical+picture), carjack (car+hijack), motel (motor+hotel), paratrooper (parachute+trooper), and many, many others. Even the word portmanteau is a portmanteau of "porter" and "manteau," meaning mantle, and refers to a type of suitcase, which word Lewis Carol then borrowed in order to coin its now more common meaning as word that packs multiple words into a single word, like Jabberwocky does.
That said, I am not in favor of all portmanteaus. Far from it. For me, in fact, some of the most insipid and insidious words in the English language are Portmanteaus that describe people's relationships, as in Brangelina (Brad Pitt + Angelina Jolie) and Bennifer (Ben Affleck + Jennifer Garner). Despite these particular assaults leveled by pop culture against my better sanity, however, I would still not toss out the portmanteau because so many words, particularly technological words like email (electronic+mail), come into English directly as a result of portmanteaus, so let's not get too flustrated by them.