1The 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.
2A severed human head, especially one taken from a defeated enemy in battle; a somewhat old-fashioned or literary usage seen in historical fiction and period dramas.
武将は敵の首を取って手柄とした。
The warrior took the enemy's head as proof of his exploit.
討ち取った首は本陣で実検された。
The severed head that was taken was inspected at the headquarters to confirm its identity.
3Dismissal; being fired — used in colloquial expressions like '首になる' (to be fired) and '首を切る' (to fire someone); often written in katakana as クビ.
彼は経費の不正でクビになった。
He was fired 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、君もクビになるかもしれないよ。
If your mistakes continue, you might get fired too.