prefix
Emphatic prefix attached to adjectives and verbs, often with a colloquial or intensifying effect. Common in words like すっごい (very amazing) and すっきり (refreshingly).
このラーメン、すっごくおいしい！
This ramen is incredibly delicious!
部屋を掃除したら、すっきりした。
After cleaning the room, I felt refreshed.
素 is a prefix meaning 'plain' or 'mere', while 素っ is an emphatic intensifier.
Derived from the prefix 素 (す) with a geminate consonant added for emphasis. The exact historical development is uncertain.