The reading ばっこ combines the on'yomi 跋 (ばつ) and 扈 (こ). The small っ (促音便) is due to 跋's original ending sound. Incorrect readings like はっこ mispronounce 跋, and はびこり is the kun'yomi of a related word. It means for bad elements or undesirable things to spread rampantly and act without restraint, dominating a situation.

跋 (ばつ) と 扈 (こ) の音読み。跋の促音便で「ばっ」となります。悪人などが勝手気ままに振る舞い、勢力を広げることを意味します。