He likely means the Cozio. Y’know. 1. e4 e5 2. Nf3 Nf6 3. Nxe5 d6 4. Nf3 Nxe4 5. Qe2 Qe7 6. d3 Nf6 where 10 points of material are going to be off the board very very quickly and the position is fully symmetrical.
What's the most defensive opening?

Ethan, thanks for correcting the name. I was referring to that line. To further correct myself, I think the Muzio gambit is a variation of the Kings Gambit?

Can anyone recommend me openings for both white and black. I'm 1700 rapid and I have been playing London system as white and Caro Kann against 1.e4 and King's Indian defense against 1.d4. All these openings are really good but now I'm getting sick of them (especially London) and want to try new ones. Thank you!
Cow opening. Opponent just has to milk you to win
When the Cow opening is more well-known than the Muzio gambit.

Can anyone recommend me openings for both white and black. I'm 1700 rapid and I have been playing London system as white and Caro Kann against 1.e4 and King's Indian defense against 1.d4. All these openings are really good but now I'm getting sick of them (especially London) and want to try new ones. Thank you!
I can offer suggestions, but I have a couple questions first: which lines of these openings do you often play (common move orders would be appreciated but not necessary), do you want to REALLY shake things up or keep it somewhat similar, and do you prefer solid or aggressive?
My initial suggestions would just be to go back to basics with e4 e5 and d4 d5 as both colors just to see if you like that style of play, but if that doesn’t sound appealing maybe try a Catalan Grunfeld French repertoire for some well traveled lines off the beaten path a ways. It’ll play more like the Caro-Kann and KID than the London, where you’ll be playing from the backseat for some really strong counterattacking chances, but all of these have dangers to them. Also, against the Slav as White, just play whatever looks decent but DON’T go for a Catalan if c6 but not e6.

Can anyone recommend me openings for both white and black. I'm 1700 rapid and I have been playing London system as white and Caro Kann against 1.e4 and King's Indian defense against 1.d4. All these openings are really good but now I'm getting sick of them (especially London) and want to try new ones. Thank you!
FRENCH DEFEEEENCEEEEEE 1.e3 as white 1.e6 as black. + stonewall, colle system, queen's indian and nimzo indian
against 1.e4 the Petrov. the Muzio line is so dry its like eating cardboard pretending to be unsalted crackers. against 1.d4 the QGA. it is a positional minefield but if navigate it right, the positions are so even that you never know who's winning.
True, but your opponent can spice things up with the cochrane gambit.
Incidentally, can someone tell me the moves of the Muzio line of the Petrov?
I'm not sure there's actually a muzio line of the petroff. however, the petroff is a generally boring opening.
That's just your frustration as white. The Petrof as black is all about stamping out white's potential attacks and swooping in if/when they make a mistake.