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1.
To experience flashing, unrelated images in frequent succession whilst in a spasmic state, such as having an orgasm or having a fit (often drug related).
"seriously dude that shit gave me a phantasmagasm"
by Ze|ane November 03, 2008
 
2.
Portmanteau of phantasmagoria and orgasm
indicates a euphoric psychedelic experience
LSD always seems to cause the best phantasmagasms
by iamslegend233 June 10, 2013