The earliest occurrence of the word "moat" that I could find online from Buffett is from 1986: https://www.berkshirehathaway.com/letters/1986.html That shareholder letter is charmingly old-school.

Unfortunately letters before 1977 weren't available online so I wasn't able to search.

It also helps that I've been to several cities with an actual moat so this word is familiar to me.

Yeah, he's been talking about "economic moats" since at least the 1990s. At least since 1995;

https://www.berkshirehathaway.com/letters/1995.html

Nobody claimed it's a new word. Still, the frequency increased 100x over the last days, subjectively speaking.