I've always liked the word dilapidated and wish people would use it in angry rants more often.

Looky-loos. Because it basically explains itself.

I also like scintillate and luminous, especially if they're both used as compliments.

Couple of my favs...

Peripatetic. Means 'of, or relating to, the philosophy or teaching methods of Aristotle.' Dunno why I like it, may have something to do with those two plosive p's in the middle, along with fairly equally-spaced consonants...

Plenitude, meaning, 'the state of being full'. Apparently the author Tolkien quite liked the word pleni*l*une, meaning 'at the time of the full moon', which is probably what made me stumble across *plenitude* in the dictionary. Again, not really sure why, but I note it's chock full of juicy consonants. Hmm, maybe I should do a scientific study of my lexical aesthetic preferences someda...ah, but I digress...

Ran into obnubilate the other day cruising the deepest dankest corners of Usenet. Just a freakin' cool word. Means, basically, 'obscure'. And obscure it is, it wasn't even in my thick Webster's. Note that I found it in its adjectival form, with the stress on the 2nd syllable, obnubilate, but it also has a verbal form, obnubilate.

In the above mentioned post, I also found such lexical monstrosities as 'adoxography', 'witzelsucht', 'sanguivorous', 'deoppilate', 'malacissitate', 'obmutescence', 'jookeries', and 'hippopotomonstrosesquipedalian'. Good words all...but, for some reason, only 'obmutescence' really resonates with me.

Penultimately, if I may be allowed to 'toot my own (proverbial) horn', in my younger years I was a frequent competitor in spelling contests, getting as high as the precursor to the National Finals (I am a resident of the USA). My family and I used to try and trip each other up with various words, a favorite being 'antidisestablishmentarianism'.

Finally, for the people who like 'ersatz', my dad recently went back to grad school and used that word in one of his most recent essays (he's trying to get teaching certification). His prof commented 'Good word!'

Cunnilingus. Oral stimulation of the female genitalia. I almost never hear this word used, yet fellatio is thrown around like nobodies business. I try to use cunnilingus as much as possible.

Additionally analingus is used even less. It's a travesty.

Another word that's always amusing to hear is ululate or ululating. And it's fun to say, too. It's basically a high-pitched screaming. Sadly, I don't hear this word used NEARLY enough.

On a side note, I didn't even know analingus was a word. Nice.

Ululate makes me think of undulate, to move in a wavelike fashion. I tend to use that one whenever something needs to move back and forth in a portentous way.

Speaking of which, portentous is another good one. I always thought it was odd in that it could mean both ominous and favorable (i.e. a good or bad portent).

Muckrake is fun to say, but I hardly ever get to use it. It means to search for and/or uncover corruption and scandal, so there's always a need for it.

Another one I've been meaning to use more is unctuous, which I first saw in Sam & Max (a great vocab builder, by the way). It means greasy or unpleasantly polite, so there are scads of great uses. I guess scads is a fine word too.

Indubitably. It makes things sound quite incontrovertible.

Being from Newfoundland, we literally have our own vocabularly toward a variety of things. I would put some of those words in this topic but since the only dictionary they exist in is the Newfoundland dictionary, I'll try to find some better words.

I use the word Gallivanting quite a lot.

Gallivanting:

1.) To roam about in search of pleasure or amusement.
2.) To play around amorously; flirt.

These aren't exactly grossly underused, since there's hardly any need or point to using them (or they're used enough) but they are a couple of my favorite words.

Floccinaucinihilipilification, a word which no longer appears in most editions of the dictionary due to its extreme lack of usage and archaic roots, is by far one of the most fun to say and spell. It means, if I remember correctly, the evaluation of something as nothing; meaningless; of little to no worth; trivial. I once used it in a conversation, accusing the poor bastard of douchebaggery (also a favorite) and general floccinaucinihilipilification in regards to my opinion of a very important topic, and he stared at me blankly like his brain had just exploded. Then he told me to spell it and I did without skipping a beat.

Pneumonoultramicroscopicsilicovolcanoconiosis, which is technically the longest word in the English language, is just a fancy medical term for black lung disease, but I love throwing that one at smokers and, far less frequently due to lack of opportunity, coal miners.

I also realized today that I love the words dongle, trepidation, and tomfoolery. Dongle is just a funny word, trepidation has always appealed to me more than its counterpart apprehension, and tomfoolery just gives such an excellent mental image of its meaning that I can't help but chuckle when I hear it.

Also, ravish - the verb, by itself. When used in context, it sounds rather sensual.

Wow, I thought hegemony is commonly used, but I guess it's only because I study social science. I would pick hegemoney instead.

Also, does abysmal counted as underused word? I think this word is even more powerful than miserable :p

I was actually wondering if someone would bring this up. I remember discovering this when I was young, [like, 7 or something] in a book about useless trivia. I've tossed it around too, but usually only when people say stupid shit like: "Oh, the longest word? Easy. Supercalifragilisticexpialidocious."

Verisimilitude is another goodie that isn't used often. It means something appears plausible.

Another great word is ergo, Latin for therefore. Some people I know think it's a confusing word and is too big a word, but therefore is technically longer. Weird.

"Impaired." Nothing too complicated that people haven't already heard before. Although that may sound too formal for normal conversations. I tend to use medical language daily, so I sometimes find myself accidentally slipping that in with my non-medical major friends without even realizing I did so.