Letha

You're all correct for the most part, save the person that thinks it's hideous. It is derived from "Lethe" and it does mean "oblivion"

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And yes, as someone that is actually named Letha. [I'm named after my gramma] I can confirm that a lot of people can't seem to understand my name. Lol. I tell them "It's like Lisa, but with a lisp." Personally, I love my name!

I seem to recall it meant "forgetful".

The name Letha itself actually is actually related to oblivion, from the name of the river through hades Lethes. I double checked this on Wikipedia (under Latis, Letha is some comic book character in a metal bra) but I first heard it from someone named Letha.