Swimming while high on marijauna.

Originally a fictional plant from Harry Potter and the Goblet of Fire, gillyweed is commonly used as a placeholder for cannabis in Harry Potter fanfiction.
Girl, I got so stoned last night at the beach, I decided to gillyweed. Water felt amazing but I had major dry mouth after from sweating.
by Herazade March 13, 2019
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A cannibalistic fish who eats anything and everything (even other fish) to have a big home all to herself.

Eeek!
There were several fish in the fish tank living with Gillyweed. Then all of a sudden there was one less fish. And then another disappeared. And finally, the last fish living with Gillyweed was found severed in two; head and tail were found floating at the top of the tank. And Gillyweed just got bigger and bigger.

Don't be a Gillyweed! Cannibalism is BAD!!!!
by The Beluga Whale November 5, 2009
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1. One that is homosexual in nature but may not be.

2. To be acting like a Gillyweed is to do stupid or uncalled-for things.

3. Gillan's nickname when he's high and silly.

1. That kid just wrote a poem, what a Gillyweed.

2. What are you doing shoplifting from the Dollar Store? You actin' like a lil' Gillyweed!

3. Gillyweeeeeeeed!

by Mike Steinhoff February 25, 2008
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