Words that don’t mean what they should:
 

indefatigable ... "incapable of being tired out; not yielding to fatigue; untiring"

“fatigable” or fatigue-able, should be “able to be fatigued”
“de-fatigable” should be “unable to be fatigued”
“in-de-fatigable” should be “NOT unable to be fatigued” or “ABLE to be fatigued”

crapulence

enbiggen.

That's a perfectly cromulent word.

I like crenelations.

casualty

nothing casual about it.

Then why are they just laying there, instead of working?

Enervating

nonplussed

I have been using "somewhat minused" in place of nonplussed.

"ensure" vs. "assure" vs. "insure". You don't know what hell is till you've sat through a 6-hour A-spec review on the relative merits of those three words.

irrespective

irregardless?

that~!

<---- irretarded