The origins of this word hark back to the invasion of the Norsemen (or vikings, to the uneducated).
Upon waking after a fruitful day of raping/pillaging Elder Boff Grimm often found that a 'batter-like' substance had been applied to the outer portion of his mouth by tiny, rotund fairies with pigtails and horned helmets during the night.
Thus, Grimm named the substance after himself (Boff) and in the absence of a true fact we can only assume the latter section of the word to be onomatopoeic, relating to the sound made upon its discovery.
Crebs: No, i'm merely sporting some bofnaj following the 14 cans off Double Dutch and Mississippi Squeeze Hog i indulged in whilst watching the 'Man o Man' Xmas special yesterday evening.
Gids: I see.