Coining up a foolish, humorous insinuation about somebody or something that is probably not valid.
Assumers are also usually, if not always assdouches. Since these unintelligent individuals seems to be associated with a lot of douchebag and asshole actions.
They are ignorant to reality, and make assumptions in order to mentally compensate for their own assdoucheyness, by creating humor for their own enjoyment, for the purposes of getting on the nerves of others.
For further information on this kind, look up "Assdouche".
"You say you don't want anybody over today Stevey? I can only make the ASSumption that you're going to have an 8 hour masturbation marathon to Jew-Porn."
"You're taking the dog out for a walk? Ha, I doubt it! I'm just going to ASSume that you're secretly a closet-zoophilliac that is waiting to act out on his urges and now this is the perfect opportunity, I'm calling the cops!"