Yet another letter of the alphabet tough for me 🙂
From the school days, X was always the unknown 🙂 the number to search for. The equation to solve.
Following that thought – probably that’s why it meant the spot where treasure was on the pirate maps? Meaning the unknown, the promise, the riddle to solve…
In Polish language, additionally, there is a saying “postawic na kims krzyzyk” (seems close to English: “to ‘x’ someone out” but not sure if completely)- to cross someone out, to give up trying to make a decent person out of someone, or give up trying to convince someone about something 🙂