I remember having learned it from the TV. But I can't remember what it is now. It's a verb synonym of drug or poison. It means someone slips some drug into someone's food or water. The drug may be just some sedatives.
I think in relation to alcohol, you may be thinking of "Slip a Mickey" or "Mick" or "Roofie".
That would possibly be "spiking" (i.e., spiking someone's drink). Hope this helps!
If the drug being slipped is a psychedelic like LSD, the word dose is often used.
Example from SF Weekly
Example from Time Magazine:
Example from The Guardian:
For the skeptics, headlines using the word "dosed" to describe people being given drugs without knowing it:
High school teacher recounts being dosed (BoingBoing)
Security guard ‘dosed’ with GHB and LSD at B.C. music festival (Canada.com)
Worst Science Article of the Week: The CIA Dosed a French Town With LSD! (Discover Magazine)
R.I.P. "Oscar": Dog Dosed With LSD By Owners Dies After Being Struck By Car (The Smoking Gun)
The Plymouth Diaries: That Time A Neo-Nazi Dosed Me With LSD (Remy Carreiro)
Family Gets Dosed With LSD From Walmart Steaks (Bubblews)
The History of "What if We Dosed the Water?" (Brainsturbator)
Johnny Knoxville Was Dosed With Ecstasy While Filming 'Jackass Presents: Bad Grandpa' Promos (Huffington Post)
And here's a "confession bear" meme somebody made: