To be unreasonable means acting in an irrational way. Your expectation that your favorite team will win the Super Bowl every year is unreasonable.

When someone's unreasonable, they're not using reason, or common sense, to make decisions or demands. Add the prefix un- to reasonable, "having sound judgment," and you get an adjective that's perfect for describing illogical behavior. It's often used for a certain kind of demanding stubbornness too: "It's completely unreasonable to make your server take your soup back five times to warm it up a little more."

Definitions of unreasonable
  1. adjective
    beyond normal limits
    unreasonable demands”
    synonyms: excessive, inordinate, undue
    beyond reasonable limits
  2. adjective
    not reasonable; not showing good judgment
    illogical, unlogical
    lacking in correct logical relation
    beyond reasonable limits
    not consistent with or using reason
    contrary to what common sense would suggest
    indefensible, untenable
    (of theories etc) incapable of being defended or justified
    mindless, reasonless, senseless
    not marked by the use of reason
    undue, unjustified, unwarranted
    lacking justification or authorization
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    reasonable, sensible
    showing reason or sound judgment
    fair, just
    free from favoritism or self-interest or bias or deception; conforming with established standards or rules
    capable of or reflecting the capability for correct and valid reasoning
    consistent with or based on or using reason
    commonsense, commonsensible, commonsensical
    exhibiting native good judgment
    healthy, intelligent, level-headed, levelheaded, sound
    exercising or showing good judgment
    tenable, well-founded
    based on sound reasoning or evidence
    well grounded in logic or truth or having legal force
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