butsu-butsu
Muttering complaints or grievances under one's breath. The words are barely audible and carry a negative, dissatisfied tone.
💡An employee after a scolding — 'He walked away butsu-butsu complaining.'
彼は不満をぶつぶつ言っていた。
He was grumbling butsu-butsu about his complaints.
He kept ___ about the unfair decision under his breath.
彼は不公平な決定について○○文句を言っていた。