{The original WOTD was, again jokingly suggested, to be “Heck”, and me being me, decided to expand the prompted to ‘all other minced oaths’, so there ya go}

Heck: it’s a minced oath of “hell” to get around censors and stuff

like tarnation is a minced oath of “tarnal”[expanded below], and damn is a minced oath of “damnation”, and crikey is a minced oath of “christ kill me”

so like, back in the real olde times and such, there was a phrase, “damnation eternal”, that was like, really bad to wish upon someone, right? but also it’s really long, so some people started saying just “damn”, as one would expect

but then also some people started saying “‘ternal”, like “eternal” but shortened, which vowel shifted to “tarnal”. and then after like 100 years, the two curses finally got joined back together in the word “tarnation”. which is really nifty cuz by that point most people didn’t know they both originated from the same phrase!

Also, jeez is just “jesus”. {And as someone else in the comments pointed out, the most “big brain” minced oath of all is “h e double hockey sticks”)