空口 reflects a Chinese eating norm: 菜 (dishes) are meant to accompany 饭 (staple grain) or 酒 (alcohol). Eating dishes 'by themselves' is mildly improper or at least informal.
usage
Also appears in 空口无凭 (kōngkǒu wú píng) — 'mere words are no proof' — where 空口 means 'empty mouth' (talk without evidence).