"I am become" is an archaic perfect (= "I have become"). Here Elon Musk's phrase parodies a quote by Oppenheimer.
Background to Parodic Reference
Robert Oppenheimer was an American physicist often dubbed the father of the atomic bomb. Following a test detonation he remarked:
... that it brought to mind words from the Bhagavad Gita: "Now I am become Death, the destroyer of worlds." (Wikipedia)
The Gita is Hindu scripture.
This is an archaic usage. For example, in the 1611 King James Bible, John 5:43 says "I am come in my Father's name". Modern translations always say "I have come". Oppenheimer's translation of the Gita likewise used the archaic construction.
In modern English, the perfect is formed using the auxiliary "have" plus the past participle.
In old texts or texts trying to sound deliberately archaic, you may find that verbs such as "come", "become", "rise", "fall" form their perfects with "be". In the case of "rise" and "fall" this could alternatively be understood as "be" plus participial adjective.
Interestingly, if you study French, German or Danish you'll find that a small number of verbs (generally including the equivalents to "come", "become", "rise", "fall") form their perfects with the equivalent to "be", while all the rest form perfects with the equivalent to "have".
- Whether Oppenheimer's translation of the Gita is accurate is irrelevant to the fact that it's become a widely cited quotation.
- There have been previous parodies of or references to Oppenheimer's quote, including "I am become Garfield" and "I am become Christmas" (the latter the name of an album).
- "Destroyer of shorts" is also a reference to Musk's attempts to frustrate the efforts of opportunistic short-term investors engaged in shorting.
- The bare noun "meme" here isn't strictly grammatical ("meme" is a count noun so we would normally expect "a meme", with an article) but the article has been omitted in order better to parallel the quote being parodied.