Crucial
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’.
wiktionary
1706, from French crucial, a medical term for ligaments of the knee (which cross each other), from Latin crux, crucis(“cross”) (English crux), from the Proto-Indo-European *(s)ker-(“to turn, to bend”).
The meaning “decisive, critical” is extended from a logical term, Instantias Crucis, adopted by Francis Bacon in his influential Novum Organum (1620); the notion is of cross fingerboard signposts at forking roads, thus a requirement to choose. [1]
etymonline
crucial (adj.)
1706, "cross-shaped, having the form of an X," from French crucial, a medical term for ligaments of the interior of the knee-joint (which cross each other), from Latin crux (genitive crucis) "cross" (see crux).
The meaning "decisive, critical, finally disproving one of two alternative suppositions" (1830) is extended from a logical term, Instantias Crucis, adopted by Francis Bacon (1620); the notion is of cross fingerboard signposts at forking roads, thus a requirement to choose.
