Why is it called whump? Why that specific word?
The Stargate: SG-1 fandom was the one that chose the word so you'd have to ask somebody there 🤷♀️ I have seen "whump!" used as an onomatopoeia for painful impacts in classic comics, so maybe they got it from that?
[Image ID: tags that read #IT WAS DANIEL #IT WAS FUCKING DANIEL JACKSON #PEOPLE LIKED BULLYING HIM SO ONE PERSON INVENTED A WORD FOR THE SOUND OF HIM FALLING OVER #AND NOW ITS AN INCREDIBLY WIDLY USED FANFIC TERM” End of ID]
☝️☝️☝️☝️☝️ yup that's pretty much it! People like Daniel getting hurt so much that the term "Danny Whumping" was coined and became so popular that the word whump is what it is now. I think other fandoms around that time (like the late 1990s) used it too but Danny Whumping was veeery popular.