This is the 348th most frequent Icelandic word.
sjálfur
"Self" or "oneself."
Here, 'sjálfur' emphasizes the individual performing the action, indicating personal responsibility.
Ég þarf að gera þetta sjálfur.
I need to do this myself.
The usage of 'sjálfir' (plural form) specifies that the group collectively performed the task without external help.
Við munum byggja húsið sjálfir.
We will build the house ourselves.
In this sentence, 'sjálf' (feminine form) highlights the fact that she alone performed the action.