420 · noun1
So named because it is 420 centimetres long.
420 · noun2
Likely influenced by the time (4:20 pm) a particular group would meet in search of marijuana, circa 1971; subsequently spread as a cipher through the San Francisco Bay area and adopted among Grateful Dead fans.
420 · noun3
From section 420 of the Indian Penal Code (and its corresponding versions in modern Pakistan and Bangladesh), which criminalizes cheating.
Etymology from Wiktionary, CC BY-SA 4.0.