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1.
A person that assumes the wrong thing all the time.
Brian Assumed Rey was sleeping with Janien .That Brian is an assumptionist.
by CortezV December 25, 2007
 
2.
Someone who assumes something, even if it is not true. An adjective that replaces assume or assumption. An assumptionist is used as an accusation.Is commonly used with such.
"Your such an assumptionist! Just 'cause I didn't do my homework that doesn't mean i'm lazy."
"What an assumptionist." "Why is she such an assumptionist? Just cause Im friends with him doesnt mean we're together."
by Djazzy April 11, 2008