Early 18th century (in the sense 'cross-shaped'): from French, from Latin crux, cruc- 'cross'. The sense 'decisive' is from Francis Bacon's Latin phrase instantia crucis 'crucial instance', which he explained as a metaphor from a crux or fingerpost marking a fork at a crossroad; Newton and Boyle took up the metaphor in experimentum crucis 'crucial experiment'.
The Latin word crux, ‘a cross’, is the source for crucial. It was originally a technical term, especially in anatomy, meaning ‘cross-shaped’, and a close relative appears in the name of the knee's cruciate ligament (late 19th century). The meaning ‘decisive’ or ‘very important’, as in ‘at a crucial stage’, can be traced back to the Latin phrase instantia crucis ‘crucial instance’, coined in the early 17th century by the English statesman and philosopher Francis Bacon (1561–1626). His metaphor was based on the idea of a signpost at a crossroad—a place where you have to choose which way to go next.
-cruc-十字 + -ial形容词词尾,性质 → 处于十字路口的