What does Isaiah 8:21 mean?
Isaiah has described those who refuse to receive God' revealed truth as living in darkness. The day never dawns for them (Isaiah 8:20). They live in this darkness and some choose to seek answers from the dead instead of from the Lord (Isaiah 8:19). This practice—called divination or necromancy—was explicitly forbidden by God (Leviticus 19:31).Now Isaiah describes them as "hungry," perhaps meaning that they are devoid of meaning. They pass through the land full of rage because of this lack. In their anger, they lash out with their words against the king and against their God. The image evoked here is a people shaking their fists at the Lord. They remain empty and unsatisfied. They dwell in darkness because they refused the light God offered to them through His revelation and the law.
It's also possible that Isaiah is describing the coming of the Lord's judgment when they will not have enough to eat. In their hunger, they will reject the Lord instead of turning to Him in humility and submission for help in their time of great need.