名詞 jackknife:

1. a large knife with one or more folding blades
   同義詞：clasp knife
2. a dive in which the diver bends to touch the ankles before straightening out

動詞 jackknife:

1. dive into the water bending the body at the waist at a right angle, like a jackknife

jack.knife

n.（名詞）
A large clasp knife.
大折刀
Sports A dive executed by jumping headfirst, bending the body at the waist, and, with the legs straight, touching the feet with the hands before straightening out to enter the water hands first.
【體育運(yùn)動】 屈體跳水，鐮刀式跳水：頭向下的一種跳水動作，把身體彎到腰部、腿伸直，在打開身體且以手先觸水的姿勢跳入水中以前，要先用手觸腳
v.（動詞）
jack.knifed,jack.knif.ing,jack.knifes
v.tr.（及物動詞）
To fold or double (something or oneself) like a jackknife.
折疊：折刀似的折攏或折疊（某物或其自身）
To cut or stab with a jackknife.
用折刀切或刺
v.intr.（不及物動詞）
To bend or fold up like a jackknife:
折起來：折刀似的折彎或折疊：
例句：
A truck that had jackknifed was blocking the road.
那輛被撞彎的卡車正堵塞著道路

To form a 90= angle.
形成九十度角