IPA: /ʌnˈstækt/
KK: /ʌnˈstækt/
Describing something that is not arranged in a pile or stack.
The boxes were left unstacked in the corner of the room.
Unstacked → It is formed from "un-" (meaning not) and "stacked" (from the verb "stack", which comes from Old English "stacian", meaning to arrange in a pile). The word "unstacked" means not arranged in a pile or not stacked.
Think of something that is 'not' ('un-') arranged in a pile ('stacked') — that's what unstacked means.