Any time you smoke weed after 4:20pm. The history as to why it is PM and not AM as well is unknown, but likely
arbitrary.
Alt: Slang for when a dealer takes a longer than usual amount of time to
deliver your pot.
"Man, it's five o'clock and you're just now
smoking up..? You're chronically late!"
or
"Usually on time, Timmy
wondered why
PK was chronically late with the kind he had ordered."