Restive

Restive [adjective]: Restless or agitated.

Don't be fooled by the root word rest.  Rather than meaning restful, restive actually means restless. It can also mean disobediently stubborn or recalcitrant. This is a fine example of one of those sneaky words that means the opposite of what you may think it means.

Tortuous

Tortuous [adjective]: Twisting, having sudden or sharp turns and corners.

Because it sounds so much like torturous, most people confuse the two. While something can be torturous, it doesn't also mean that it's tortuous (or vice-versa). For instance, a book with many twists and turns can be tortuous, but quite an exciting and engaging read. A torturous book, on the other hand, is just unpleasant to read.

Fungible

Fungible: adjective: Replaceable of exchangeable for a similar item.

Though not as obscure as other strange English words, fungible looks deceptively like fungus and edible, which might put you in the mind of edible fungi. This word is actually an economic term meaning replaceable or exchangeable for another of its kind.  Currency is a fungible commodity; you can use any $20 bill to pay for a $20 item.

Fagin

Fagin (noun): an adult who instructs and involves others (usually children) in crime.

This is a prime example of a character's name from a novel that became a word. Fagin is a petty criminal in Charles Dickens' "Oliver Twist" who manages and trains a band of thieving young boys.

Eutrapely

Eutrapely (noun): Pleasantness in conversation. This is one of the seven moral virtues that Aristotle enumerated.

Is it a sign of the times, perhaps, that this word is now obsolete whereas there are no shortage of words to describe unpleasant conversation?

Zoilist

Zoilist (noun): A person who thoroughly enjoys finding fault with others or in things around him or her.

Say the word out loud to yourself now that you know the meaning and see how many people immediately come to mind (except yourself, of course). That's the really interesting part about it: no true zoilist ever considers him or herself to be one.

Jumentous

Jumentous: Pronounced /dʒuːˈmɛntəs/: Smelling distinctly like horse urine.

Why someone saw fit to create a word that specifically identified horse urine when even humans don't have that consideration remains a mystery. It's also a rather tricky word because it sounds positive and glorious. Imagine being told that you are jumentous (but not knowing the meaning). You would walk away confident that you have an admirer of some sort.