From 一(いち) (ichi, “one”) and ころり (easily; just like that), as they are used in phrases like いっぺんでころりと負(ま)ける (ippen de korori to makeru) or 一(いち)撃(げき)でころりと倒(たお)れる (ichigeki de korori to taoreru).
いちころ or イチコロ • (ichikoro)