IPA: /ˈspeɪsiəst/
KK: /ˈspeɪsiːst/
Having the most space; very spacious or roomy.
The new apartment is the spaciest one I've ever lived in.
Comparative: spacier
Superlative: spaciest
The word 'spaciest' is derived from 'space' (from Latin 'spatium', meaning room or extent) and the suffix '-iest' (meaning most). Therefore, 'spaciest' refers to having the most space or being the most spacious.
Think of 'space' as room or area, and remember that 'spaciest' means having the most room.