And when I say "Doo-doo, doo-doo,"

from ScHoolboy Q (Ft. Kendrick Lamar) – Collard Greens Lyrics on Rap Genius

Meaning

This is Kendrick’s signature ad-lib likely developed from his “Cartoons & Cereal”. This ad-lib is meant to resemble gunshots. This is something Kendrick came used to due to his upbringing in Compton, a city known notoriously for its high crime rate.
The same onomatopoeia he used on
“The Art Of Peer Pressure,”
“1 Train,”
“The Jig is Up,”
“Fragile.”

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