OPPOSITES ATTRACT

A few words which mean the opposite of how they sound:

pulchritudinous - should mean: horribly covered in boils; does mean: very beautiful.
hagiography - should mean: a biography tearing the subject's character to shreds; does mean: an over-praising biography.
persiflage - should mean: a stream of persecutory invective; does mean: playful banter.
dilate - should mean: of an aperture, to grow smaller; does mean: to spread out or expand.
droll - should mean: sad and humourless; does mean: cleverly witty.
inflammable - should mean: incapable of burning; does mean: easily set alight.
disembark - should mean: to leave port, to go on a journey; does mean: to leave a ship.
embark - should mean: to enter port, return from journey; does mean: to get on a ship.
spendthrift - should mean: one who is careful with money; does mean: one who spends freely and wastefully.
dirigible - should mean unmoveable, tied to earth; does mean: navigable, as of an airship.

Source: list by John Stammers

Roddy adds:

numptious - should mean: unpleasant and arrogant; does mean: delightful and cuddly.
cataglottal - should mean: bunged up with phlegm; does mean: pertaining to passionate kissing with tongues.
wiseacre - should mean: a bright person dripping wit and wisdom; does mean: a naive fool.