That is to say, ‘cent’ means 100; ‘percent’ means ‘of 100’.
Centum = latin for hundred. I believe there’s an adjective, something like centesimus, that means “hundredth.”
Originally (and odds are samclem and I are the only two Dopers who know it), the $0.10 coin was a “disme” – coined from decimus, one-tenth. The first, pattern pieces were called “dismes.” The “-s-” infix was dropped within the first year, and all ten-cent pieces since then are “one dime.” Unlike other current American coins except the cent and dollar, it’s the official, legal name for the coin.
Until 1857, the U.S. issued a tiny silver coin called a “half dime,” worth five cents in silver. Beginning with a pattern piece in 1856 and continuing to the present, this was replaced with a larger, baser-metal coin which was originally made of a mostly-nickel alloy. Hence “nickel” for the five-cent piece.
The quarter is of course a quarter dollar, $0.25. And the half-dollar needs no explanation.
Prior to 1933, we had gold coins: a tiny gold dollar, and four standard coins valued at $2.50, $5.00, $10.00, and $20.00 – respectively the quarter eagle, half eagle, eagle, and double eagle. (There were also, for a while $3.00 and briefly $4.00 gold coins.)
A fair amount of British money slang comes from the period before decimalization (early 1970s) where a penny was 1/240 of a pound, 12 pennies to the shilling and 20 shillings to the pound. Not to mention that the Brits insist on stating prices in “guineas” – which have not been coined since, I think, 1797. 
From here (emphasis mine):
I’d always assumed that the U.S. ‘cent’ came from the French centime (which would have come from the Latin centum).
Huh? We do?
Guineas, as a unit of currency, went out with decimalisation in 1971. They were a bit of a scam anyway. One guinea was 21 shillings, equivalent to £1.05 in today’s money. Expensive items were priced in guineas to make them look cheaper. A price of 200gns seemed less than £210 somehow.
Only in a few places; I believe horses are still bought and sold that way.
I figure I might as well add a few more coin slang, this time from the Canadians, for those of you who might be curious. Often we will use the same terms as Americans, but we have a few of our own, too. I’ll add French terms for those of you going to Québec one day! If I missed anything, I apologise!
1 cent = “cent”, “penny” or, in French, “sou”, “cents” but pronounces “senne”
5 cent = “nickel”, French = “cinque cents” or “cinq sous”
10 cent = “dime”, French = “dix cents” or “dix sous”
25 cent = “quarter” French “vingt-cinq cents/sous”
1 dollar = “loonie” (so named because it has a picture of a loon it. The Queen appears on the other side, as with all coins), sometimes called a “buck” (after the American term) or simply a “dollar”. French “un dollar” or “une piastre”
2 dollar (it’s a coin here) = “toonie”, derived from “loonie” only now it’s 2! French = “deux dollars/piastres”.
Back in 1996, when the toonie was first distributed, there was a lot of talk about what Canadians would call it. One popular example (at least in my home town) was the Moonie. Why?
because it had the Queen in front with a bear (bare) behind!
I agree; however, Spingears appears to be suggesting ‘cent’ is a contraction of ‘percent’.
To the best of my knowledge guineas are still coined every year…a limited number maybe, but still…
Also as has been stated horses are sold by the guinea but I have a feeling that this is just an archaic thing.
I also believe we still issue “Groats” (4 pence) coins but for what reason I haven’t the foggiest unless it has something to do with Maundy money which ER2 dishes out each year to deserving people or causes.
I’ve never bloody had any and I deserve some 
Ah Pride, thy name is Polycarp. 
As to dime, from here.
“The name of the coin comes from the French disme (modern French spelling dîme), meaning “tithe” or “tenth part,” from the Latin decima [pars]. This term appeared on early pattern coins, but was never used on any circulating dimes.”
I’ve never heard of this, and I used to collect coins. Do you have a cite?
The “groats” are Maundy money, but they aren’t really groats as their face value is 4p, that is, 4 decimal pence, not 4d (old pence). There are also Maundy 3p pieces.
Oops, accidentally posted too soon.
There are four Maundy coins, for 4p, 3p, 2p and 1p. They are all made of silver and the 2p and 1p coins bear no resemblance to the ordinary 2p and 1p coins. You’ll never see any of them in circulation.
Here’s a link to photos of a 1792 half disme. About 1500 of these were struck.
The three cent silver piece was sometimes called a “trime”. How often the term was actually used in everyday speech, I don’t know. And I HAD heard of the “disme”.
Just wanted to say that this is correct. You might pick up a penny from the sidewalk, but a price would always be expressed in cents.
And “dime” (and yes, I knew “disme”, too, though I probably learned it from samclem) is officially a unit of currency (the coin says “one dime” on it, not “ten cents”), but it’s never actually used as such, only as the name of the coin. You’d never say something costs “two dimes”, you’d say “twenty cents”.
Officially, the US also has a monetary unit called the “mil”, equal to one tenth of a cent, but I don’t think there have ever been coins in that amount (there certainly aren’t now), and they’re only ever used in a few specialized contexts.
If we’re keeping track I knew a dime was originally spelled disme and was an official unit, but …
I’m mostly here to report that one-thousandth of a dollar is a mill (with 2 l’s). A mil is one thousandth of an inch mostly used to measure thickness. See e.g.,
A mil is 1/1000 of the Hong Kong dollar though
::: repents in sackcloth and ashes from being pelted with dismes by irate Dopers :::: :o
I tried to popularize “doubloon” (as it was a double loonie, ye see, arrrrh!), but it never caught on. There are advantages to being able to talk about a loonie-toonie fiscal policy, however