Zonked out is a common expression to describe someone who is completely exhausted or tired. However, there are several synonyms for zonked out that can be used to express the same thing. For example, you can use the phrase "spent" when referring to someone who is completely fatigued. "Beat" is another synonym that could be used to describe someone who is worn out or knackered.
Other synonyms of zonked out include "drained", "wiped out", "exhausted", "dead tired", and "completely bushed". Using synonyms can help add variety to your vocabulary and prevent repetitive language from making your writing or speech dull and uninteresting.