Origin of the word "yeet"
This was going to be a comment but it wouldn't fit.
Read this SMH article for a detailed description:
Wiktionary says is was used as early as the 2000s but a viral Vine video popularized it in 2014. As interjection it can mean anything from expressing excitement, satisfaction, nervousness. Additionally it's listed in Urban Dictionary and Wiktionary as both a noun and verb. Both as noun and verb it can mean many things. It probably has even more meanings as a verb, ranging from to throw, hanging out, messing around, violating rules or laws, etc. The "throw" meaning is attested in the top definition of Urban Dictionary, and in Wiktionary.
Some user 11 months ago commented on the Sydney Morning Herald article:
"Yeet" was originally concocted as an elision of "Yes!" (with the exclamation mark) and "Neat!" (also with the exclamation mark). The obvious Americanism, "neat", underscores the US origin of the term.
I have no idea what this claim is based on.
I have a feeling that beyond the exclamation/interjection use of it there's not much of a consensus on its meaning. Here is a Google Trend chart on its incidence starting from 2004. It starts to take off about January 2014, which would corroborate the claims of Wiktionary and Know Your Meme that it spread from Vine and Youtube videos of people doing a dance. Yeet also means this type of dance, but from what I've seen they look like different dances.
My favorite definition from Urban Dictionary is the fourth one:
n.Everyone thinks they know what it means until they realize they have no f*#k!ng clue.
It's interesting you refer to "yeetus", because "yeetus" is defined in Urban Dictionary as someone who can yeet, and this definition was posted a month after the earliest "yeet" entry, which is given as an expressive exclamation. Interestingly the earliest Urban Dictionary entry for "yeet" is from 2017, and although Wiktionary has a quote from 2014, I believe it may be referring to the dance meme. The rest of the quotations on Wiktionary not directly referring to the dance meme also start in 2017, so I think 2017 may have been when it's meanings started to drift away from the dance meme topic.