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Sleeping, and not in the sexy way; Falling asleep.
"Man, I was so tired last night, I was catching Zs in no time!"

"You could sure tell he was catching Zs, I could hear him snoring all the way down the hall."
by Codie Renee July 14, 2008
To sleep, rest, or take a nap
I was catching z's during study hall.
by Jeff Carter January 17, 2005