The onager was a post-classical Roman siege engine, which derived its name from the kicking action of the machine, similar to that of an onager (wild ass). It is a type of catapult that uses torsional pressure, generally from twisted rope, to store energy for the shot.
Twisted rope? Now I've gotta read the rest of the article.
Oh, I forgot to tell you why he asked.
He's a tree trimmer, you know, and he was removing a big maple today, cutting all the branches off and dropping them to the ground. When he cut off the last and biggest "lead" (think of a lead as a sub-trunk), a branch hit the ground funny and shattered, shooting a yard-long, five-inch thick, jagged log at a neighboring house. Luckily, she, the neighbor, had a big maple tree of her own that stopped the log before it sailed right through her picture window.
He said it all happened in slow-motion, but it didn't get a good description of his feelings. He went off on a tangent about what an SOB his father-in-law is.
He probably tells his father-in-law what an SOB his stepdad is. I know I can be an SOB. I don't know anything about the other guy but what my stepson tells me. I stepped into this situation with no clue of what I was getting into, thus modeling the lifestyle I obviously favored. The Older Boy has gone and done likewise.