Self-Antonyms / Janus Words / Contronyms
One of the coolest and smallest classes of words is those words that are their own antonyms;
that is, words that have both a meaning and the opposite meaning.

These words are an instance of polysemy, in which a single word has multiple meanings. In the special case of contronyms, the polysemous terms contain two opposite meanings.
Here are some examples, most in common English usage (if you know of others, please email me at teller at

Note:  I have omitted slang usages, such as "yeah", "right", "bad", and so forth.
sanction means "to allow" and "to prohibit (or punish)"

cleave means "to join" (as in "cleave unto") and "to separate or divide"

clip means "to attach" and "to separate" (sure looks like clip is a cognate of cleave, doesn't it?)

inoculate means "to protect against" and "to infect with"

cull means "to select" and "to reject"

alight means "to settle onto" and "to dismount from"

went off means "to start" and "to stop" (the alarm ~ when the light ~)

off means "inside of" and "outside of" (we compress the data ~ the camera)

fix means "a solution" and "a problem" (also fixed)

cool means "supportive of" and "opposed to" (he was "cool with" the idea; he was "cool to" the idea); sort of a slang usage

public means both "public (free)" (in American English) and "private (fee-based)" (in British English)

ravel means "to entangle" and "to disentangle" (as does unravel!)
    [contributed by Tamara Munzner]

screen means "to display" (~ a movie) and "to hide" (~ his view)
    [contributed by Krishnan Sriram]

protest means "to object" and "to affirm" (also, protestations)
    [contributed by Ron Slavecki]

cork means "to take out" and "to insert" a cork from a bottle
    [contributed by David Miller]

oversight means a kind of error, and a kind of prevention from error
    [contributed by Matt Ross]

trim means "to remove from" (~ the tree) and "to add to" (~ the Christmas tree)
    [contributed by Dick Stadler]

enjoin means "to direct" and "to forbid"
   [contributed by Jonathan King]

dust means "to remove from" (~ the table) and "to add to" (~ the cake)
   [contributed by Susan Ramage]

clip means "to partition" (~ the paper) and "to join" (~ the papers)
   [contributed by Sidney Pestka]

secrete means "to give off" and "to conceal"
   [contributed by Matt Antone]

rent means "to grant possession in exchange for rent" and "to take and hold under an agreement to pay rent"
   [contributed by Matt Antone]

can means "to save" (~ the peaches) and "to discard" (~ the worker)
   [contributed by Sylvia Briggs]

belie means "To picture falsely; misrepresent: 'He spoke roughly in order to belie his air of gentility' (James Joyce)." and "To show to be false: 'Their laughter belied their outward grief.'"
   [From; contributed 12/05 by Nick Serafin]

stem means "To start or originate" and "To stop or restrain"
   Contributed 12/07 by Michael Solomon]

peruse means "To read in an attentive manner" and "To read in a leisurely manner"
   Contributed 8/2011 by Wayne Yang]

draw means "To bring together" and "To pull apart" as in "Please draw the curtains"
   Contributed 9/2012 by Rae Langton]


Kate Curtis-Mclane contributed a whole pile of self-antonyms:

settle means "to move" (the pile ~d) and "to stop moving" (we ~d in)

    flesh means "to add substance (~ out)" to and "to clean a hide of flesh"

    seed means "to put seeds in" and "to take seeds out"

    garnish means "to add something to" or "to take away from (a form of ~ee)"

    root means "to get something to take root" or "to pull up (root out)"

    joint means "to combine or attach with a joint" and "to separate (esp. meat) at a joint"

    snap means "to break into pieces" and "to fasten together"

    tube means "to insert a tube in" and "to enclose in a tube"

    reel means "to wind onto" and "to let out from"

    lease means "to pay for use" and "to be paid for use"

    water means "to pour water out" and "to take on water"

    wear means "to last under use" and "to erode under use"

    weather means "to disintegrate or wear" and "to come through safely, survive"

    crop means "to plant or grow" and "to cut or harvest"

inflammable and flammable both mean "capable of burning".
    This isn't a self-antonym but it is a cool word pair.
If you know of other examples, please email me at teller at