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?

Jimmy: BEEP BEEP LETTUCE

Person 1: ...
by serialspongedesecrator September 16, 2017
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The most annoying sound in the world. Whether it's censorship, an alarm clock or a microwave, its always annoying.
ALWAYS
Alarm Clock: *BEEP* *BEEP* *BEEP* *BEEP* *BEEP* *BEEP*
Me: uuuuuuuuuuugh shut UP
by CURNICNLGEE February 15, 2021
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BEEP
BEEP
by XaviBear2012 September 2, 2021
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a word to replace another word. Can be used as a noun, verb, or adjective.
What’s the beep?”
I’m on my way beep from beep.”
“Ok drive beep!
by DJ young kev May 7, 2021
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Hi I’m Keith I speak robot like this " Beep bop boop skdoo doo bep "
by Call Meh Lemon Ayla May 20, 2021
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