butsukusa
Muttering complaints or grievances under one's breath, usually in a discontented way. Not meant to be heard clearly by others.
💡A student complaining about homework — 'He was butsukusa grumbling about the assignment.'
彼はぶつくさ文句を言いながら片付けた。
He cleaned up while grumbling butsukusa.
She was ___ muttering complaints while doing the dishes.
彼女は皿洗いをしながら○○と文句を言っていた。