盆暗 【ぼんくら】 (n) (1) stupidity; idiocy; (2) blockhead; idiot; dimwit; mediocrity [G][GI][A]
not "Bonklers" even if old fansubs did say that

They actually did add -zu, making it into an impromptu plural.

For more recent comparison, think of Futami Ami & Mami in The Idolm@ster, who perform under the stage name Ami's. Randomly adding on English plurals to Japanese text isn't common, but it happens.