1. the putative use of the inside of a banana peel as a halucinagenic drug (a 1960's hoax that became connected with an urban legend about a song of a similar name after the debut of the said song, and susequently spawned more hoaxes, sense 2 is likely also linked to the song)

2. Lysergic acid diethylamide, a hallucinogenic drug

3. a five milligram tablet of valium (from the yellow color of the tablet)
I wouldn't waste my time with mellow-yellow if I were you.
by The Return of Light Joker March 21, 2011
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Crashing after doing some sort of drug
StonerBoy: "You Still High?"
StonerGirl: "Nawh, I'm mellowing out now."
by ThatGirlWhoDatedThatOneGuy January 27, 2011
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A joint/blunt rolled with "mello mango" (or any other mango flavored rolling papers), marijuana, any non-pot cannabis related substance, crushed up percocet and the tobbacco inside a Marlboro Smooth menthol cigarette
Hey man, we need to enjoy the scenery more. Let's mellow out. You know man, really get in touch with nature. We need to smoke a Mellow Yellow man. Listening to Led'll do that to you man.
by Tricky Stevensen June 14, 2010
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When you're fucking a bitch who is so ugly that you can't get hard, so you pee in her.

see sour puss
I gave that ugly bitch a mellow yellow last night. Then I beat the fuck out of her.
by Rev. Reggie Dingleberry August 18, 2004
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During sex, casually pissing inside a girls beefcave(vagina)
a guy and girl are in the middle of sex and he gives the girl a mellow yellow
by eat my boogers October 15, 2010
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