To "Phelps" a joint or bowl of marijuana is to finish it in under one minute. After finishing, the person who takes the final hit must declare "I told you guys... under a minute" This term stems from Michael Phelp's Rosetta Stone ad in which he completes the ad in under a minute and says the aforementioned quote. This is combined with the new information that Phelps is a raging potaholic.
We're late for the movie man! Phelps that shit! *59 seconds later* "I told you guys... under a minute."