This saying is originally from a rather obscure meme that includes Max and Ruby. At the end of the comic, Max exclaims "BEEP BEEP LETTUCE" in response to a question Ruby asks.
Essentially, Beep beep lettuce can be used as an interjection whenever one feels it's appropriate or just want to mess around with whoever they are with.
Person 1: Just grab me the Ragu from the stock room, ok Jimmy?
that song you just can't remember the name of. And it makes your friends think you are a robot
Guy 1: Hey can you play that song again? it went beep boo boo bop boo boo beep.
Radio Disc Jockey: Nah man you mean the song that goes beep boo boo bop boo boo bop
boyfriend from friday night funkin' says this when lemon demon tells him he will separate him from his eyes, turn his girlfriend inside out and burn her finger nails
lemon demon: separate you from your eyes, turn your girlfriend inside out and burn her finger nails la la la la
bf: beep bap boop boop be bo ep bo bep bap ep op eep bap bop bep bap boop beep boop beep bap