Firing your wad or shooting your wad is an old muzzle-loading musket term.
It’s funny that outdated terms tend to shift meanings to popular ‘modern’ ideas for whatever context of modern we’re talking. So in the seventeen and eighteen hundreds, a lot of mundane terms that were then outdated became religious, spiritual, or mystic in connotation. Nowadays, things tend to become sexual.
It’s not that our old-timey ancestors were less sexual than we are, but we have fewer universally relatable concepts. This is due to both our generally increased world-spheres and more diverse social circles. Someone in 1700s Massachusetts would spend a higher percentage of their time talking to other people from Massachusetts, probably close to 100% for most people, than today. What’s more, those people would pretty much all be white Puritans. Now my friend circle crosses the Atlantic, is localized in much of the West, and hits people of varying religions, political beliefs, and so on. We don’t all have religious commonalities to fall back on. We have biology, and that’s eating, drinking, sleeping, and sex. And dragons.
The process of contraction is roughly random, so if you’ve got religion, locality, biology, and dragony in common with people, and say sex is 1/4 of biology (conversationally, whatever), 1/16th of shifting expressions would have landed on sexual. Nowadays religion and locality are far smaller, so say their relevance drops by half. So 1/3 (1 / (bio + 0.5 religion + 0.5 locality + dragons) of terms shift to biology and of those, 1/4 are sexual, giving a final proportion of 1/12. 1/12 is greater than 1/16. All numbers completely made up, but I’d be interested in historical etymology if someone has the statistics.
Anyway, it’s odd how languages change.