If you're a straight-A student and still you worry about failing all of your classes, you're being irrational. Your fears are not based on fact and not likely to come true.

Usually you use irrational to describe ideas and thoughts that are not based on reason, although emotions and behavior can be irrational too if they don't seem reasonable. Do you jump onto a chair and scream when you see a mouse? If so, you have an irrational fear of mice, or musophobia. If someone takes a swing at you for failing to say excuse me when you burp, both their anger and their actions could be called irrational.

Definitions of irrational
  1. adjective
    not consistent with or using reason
    irrational fears”
    irrational animals”
    without logical or meaningful connection
    illogical, unlogical
    lacking in correct logical relation
    blind, unreasoning
    not based on reason or evidence
    not endowed with the capacity to reason
    not based on reason
    showing ignorance of the laws of nature and faith in magic or chance
    not reasonable; not showing good judgment
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    consistent with or based on or using reason
    capable of or reflecting the capability for correct and valid reasoning
    coherent, logical, lucid
    capable of thinking and expressing yourself in a clear and consistent manner
    demythologised, demythologized
    having mythical elements removed
    intelligent, reasoning, thinking
    endowed with the capacity to reason
    reasonable, sane
    marked by sound judgment
    reasonable, sensible
    showing reason or sound judgment
    mentally healthy; free from mental disorder
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  2. adjective
    real but not expressible as the quotient of two integers
    irrational numbers”
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    capable of being expressed as a quotient of integers
  3. noun
    a real number that cannot be expressed as a rational number
    synonyms: irrational number
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    transcendental number
    an irrational number that is not algebraic
    algebraic number
    root of an algebraic equation with rational coefficients
    the ratio of the circumference to the diameter of a circle; approximately equal to 3.14159265358979323846...
    the base of the natural system of logarithms; approximately equal to 2.718282...
    type of:
    real, real number
    any rational or irrational number
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