PTS Pali-English dictionary - The Pali Text Society's Pali-English dictionary
Huhuṅka
Huhuṅka，（adj.） [fr．huṁ] saying “huṁ，huṁ，” i．e．grumbly，rough； °jātika one who has a grumbly nature，said of the brahmins Vin.I，2； Ud.3 （“proud of his caste” Seidenstūcker）．nihuhuṅka （=nis+h.） not grumbly （or proud），gentle Vin.I，3； Ud.3．Thus also Kern，Toev．I.137； differently Hardy in J.P.T.S．1901，42 （“uttering & putting confidence into the word huṁ”） Bdhgh （Vin.I，362） says：“diṭṭha-maṅgaliko mānavasena kodhavasena ca huhun ti karonto vicarati.” （Page 732）
Buddhist Dictionary of Pali Proper Names by G P Malalasekera
Huhunka
Huhunka：The descriptive name of a brahmin who visited the Buddha at theAjapālanigrodha seven days after his Enlightenment and asked him questions regarding the true brahmin．The Buddha explained the matter，but made no impression on him （Vin.i.2； see also Ud.i.4； UdA.54； cf．Mtu.iii.325）．
Buddhaghosa explains that the brahmin was a ditthamangalika，filled with haughtiness and wrath，and went about uttering the sound ”hu-hum.” hence his name.