The term "420" is heavily associated with marijuana culture. Despite the term's popularity, its origin is unknown and widely debated. Definitively finding the term's original use would put a decades-old cultural debate to rest.
420 is not actually a police code for cannabis, but it is a police code for murder in Nevada. In urban legend, it is police code for murder in more states. As for how that lethal police code way gave to 420, marijuana was made illegal in the United States after “reefer madness,” or an anti-marijuana cultural movement, worked for its use to be criminalized. They disseminated propaganda stating or implying things like marijuana causes Black men to rape and murder.  Since murder became linked with marijuana during the era of reefer madness, this is the historical foundation for modern 420 being linked with police code for murder. The two situations form an analogy, with murder itself today and the falsity that Black men murder after consuming marijuana sit parallel. The side of the analogy modern persons focus on is that marijuana and 420 are also parallel, and therefore connected because murder and marijuana are connected historically, although most are unaware of the other half of this phrase. Despite this being the true origin of 420, the general public believes 420 is linked to marijuana because it is the penal code for possession. Although these people are misinformed, what they realize is that 420 is a celebration of marijuana in the face of oppression regarding its consumption, since it is criminalized in most countries. By associating 420 with a penal code, albeit the incorrect one, they understand the meaning behind the actual link of 420 and murder. Both are about the police persecuting marijuana consumption (and thereby consumers). Both those informed of the true origins and those who believe 420 is penal code for possession celebrate 420 as a way of turning their condemnatory code into a badge of honor, thus upsetting the oppressive conditions they live under 364 other days of the year. Thus, 420 arises from the actual police code for murder and the idea in the collective mind that 420 is police code for cannabis. 420 has a legal origin in police penal codes, whether they are real or imagined. Its original use is unknown since it is a slang term that spread by word of mouth, but its strong ties with police penal code prove that it originated in this way.
420 is police code for murder in only one state. Furthermore, there is no knowledge of who coined the term in this clever origin story. When did Nevada start using 420 as code for murder? Why would murder and marijuana become linked before the penal code for drug possession and marijuana? This argument seems unlikely, has a lot of holes, and is little known. The fact of the matter is 420 is not police code for smoking cannabis in any state. This convoluted explanation of murder being the bridging link between 420 and marijuana has no evidence to back it up, and sounds more like a constructed history lesson than an actual origin story.
[P1] 420 was originally the police code for murder. [P2] Cannabis is linked to murder due to "reefer madness." [P3] Therefore, the meaning of 420 came from the association with the police code.