Antonyms of adj reasonable

3 senses of reasonable

Sense 1
reasonable (vs. unreasonable), sensible -- (showing reason or sound judgment; "a sensible choice"; "a sensible person")

unreasonable (vs. reasonable) -- (not reasonable; not showing good judgment)
        => counterintuitive -- (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; "mindless violence"; "reasonless hostility"; "a senseless act")
        => undue, unjustified, unwarranted -- (lacking justification or authorization; "desire for undue private profit"; "unwarranted limitations of personal freedom")

Sense 2
fair, fairish, reasonable -- (not excessive or extreme; "a fairish income"; "reasonable prices")

INDIRECT (VIA moderate) -> immoderate -- (beyond reasonable limits; "immoderate laughter"; "immoderate spending")

Sense 3
reasonable, sane -- (marked by sound judgment; "sane nuclear policy")

INDIRECT (VIA rational) -> irrational -- (not consistent with or using reason; "irrational fears"; "irrational animals")

Similarity of adj reasonable

3 senses of reasonable

Sense 1
reasonable (vs. unreasonable), sensible -- (showing reason or sound judgment; "a sensible choice"; "a sensible person")
       => commonsense, commonsensible, commonsensical -- (exhibiting native good judgment; "arrive home at a reasonable hour"; "commonsense scholarship on the foibles of a genius"; "unlearned and commonsensical countryfolk were capable of solving problems that beset the more sophisticated")
       => healthy, intelligent, levelheaded, level-headed, sound -- (exercising or showing good judgment; "healthy scepticism"; "a healthy fear of rattlesnakes"; "the healthy attitude of French laws"; "healthy relations between labor and management"; "an intelligent solution"; "a sound approach to the problem"; "sound advice"; "no sound explanation for his decision")
       => tenable, well-founded -- (based on sound reasoning or evidence; "well-founded suspicions")
          Also See-> fair#1, just#3; logical#1; rational#1; valid#1

Sense 2
fair, fairish, reasonable -- (not excessive or extreme; "a fairish income"; "reasonable prices")
       => moderate (vs. immoderate) -- (being within reasonable or average limits; not excessive or extreme; "moderate prices"; "a moderate income"; "a moderate fine"; "moderate demands"; "a moderate estimate"; "a moderate eater"; "moderate success"; "a kitchen of moderate size"; "the X-ray showed moderate enlargement of the heart")

Sense 3
reasonable, sane -- (marked by sound judgment; "sane nuclear policy")
       => rational (vs. irrational) -- (consistent with or based on or using reason; "rational behavior"; "a process of rational inference"; "rational thought")