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1.
A combination of fantastic and orgasmic/orgasm.
Its meant to overly stress how great or wonderful something is.
If the Steelers win the super bowl that would be fantasm!
by Pixxiedame February 14, 2011
 
2.
verb; to experience a fantasy, usually used in conjuncture with broken English.
"every night i fantasm on girls"
by Jasper Gilispie April 25, 2010
 
3.
A fantasm occurs when one ejaculates (most likely while being felated) while simultaneously passing a log.
"I just had a phantasm, and let me tell you, it was something else."
by Chris March 14, 2005