If you saw my post a few weeks ago on excruciating/crucifixion, you might just look at the word crucial and say, “Well, huh. That has that cruc root in it too!”
And you’d be right. Crucial also has the same root, which literally means “cross” in Latin. But in the case of this word, we actually owe Francis Bacon thanks for our meaning of “critical, of the highest importance.”
You see, in addition to being a torture device, a cross was also a very simple form used for practical things like signposts. In his work, Instantius Crucis in 1620, Bacon takes the literal signpost and its Latin word and uses it metaphorically–when you see a signpost, you know to pay attention, right? Following the right direction will be of the utmost importance to where you end up on a journey.
Well, by the 1730s English had adopted the literal meaning of crucial–shaped like a cross. And by 1830, the metaphorical meaning had come along too. I always find it interesting when the later, symbolic meaning has completely overtaken the literal one in modern speech!