This is the 228th most frequent Japanese word.
いっ
"いっ" is not a standalone word in Japanese but could be a part of words or expressions. Context is necessary for definition.
Here, 'いっ' is part of 'いっしょう', which means 'with full devotion' or 'maximum effort'. It intensifies the phrase.
いっしょう懸命に勉強しています。
I am studying with full effort.
In this example, 'いっつも' is a colloquial variation of 'いつも', meaning 'always'.
彼はいっつも元気です。
He is always cheerful.
In 'いっさい', 'いっ' helps indicate 'absolutely' or 'entirely', often for phrases that negate something or emphasize entirety.