The neck — the part of the body connecting the head and torso.
首が痛くて振り向けない。
My neck hurts and I can't turn around.
寒くて首にマフラーを巻いた。
It was cold, so I put a scarf around my neck.
首を曲げて本を読むのは目に良くない。
Bending your neck to read books isn't good for your eyes.
Head; chief — used figuratively or colloquially to refer to the leader or person in charge.
この小さなチームの首は誰だ？
Who is the head of this small team?
現場の首が方針を決めた。
The person in charge on site decided the policy.
Dismissal; being fired — used in colloquial expressions like '首になる' (to be fired) and '首を切る' (to fire someone).
彼は経費の不正で会社を首になった。
He was fired from the company for expense fraud.
業績不振で数名が首を切られた。
A few people were let go due to poor performance.
もしミスが続けば、君も首になるかもしれないよ。
If your mistakes continue, you might get fired too.