This filler word originates from the vocalisation used by the Shocker Combatmen, the foot soldiers of Shocker, an evil organisation striving for world domination in the Japanese live-action series Masked Rider.
These combatmen are recognisable by their distinctive black outfits with skeleton patterns and their black-and-white masks adorned with the Shocker logo.
In the original Japanese series, the combatmen vocalised a distinct “Eee-Eee” sound. However, Thai voice actors mistakenly altered it to “Ki-Ki.”
As a result, the term has come to specifically refer to expendable henchmen who are willing to sacrifice themselves for their leader’s goals—though their efforts are often futile, as the superhero ultimately prevails.
AloJapan.com