Is there such a thing?

If someone offers me a bet for $500, saying that English is a Latin language and I am 100% positive that English is an Indo-European language through German, I am not going to accept that bet. I don't consider that to be a bet because I would just be taking his money. If I were only 75% certain I might take the bet.

If I can tell handedness by watching a person walk(they have a slight lead with dominant side) and get someone(who doesn't know what I know) to bet me and give me odds due to ratio of left:right handedness(subject appears left handed) I would feel very dirty even tho the persons walk could have been due to a slight limp caused by injury, etc, making me only ~90% certain the subject is left handed.

Is there friendly betting and exploitative betting?

In the Stock Market it's illegal to trade on insider information.

What you describe is not inside information. It's just an information advantage that you use to exploit others. Same as the goal of any other bet, investment, and many times in sales.

Insider information is exactly that. Insider. Information not everyone is privy to.
Everyone has the opportunity to be aware of language derivation. If they choose not to become informed, that's their loss.

English, like many languages, is a mix of many languages. Latin is certainly one ingredient. You can't pin it down to a single source.
Bet is a push.

How is this different from playing poker against someone whom you know you will beat in the long run? Is this unethical as well?

Everyone has access to learning how to outsmart people / bet with a positive edge on things such as winning at poker or chess or golf etc, not everyone can know behind the scenes info secretly - think if lodden thinks took place when lodden tells one of the people his actual number before hand secretly = scamming.

Like betting on the river with the nuts?

In your left hand/right hand scenario a comparable to insider trading would be if you knew the person or saw them using their dominant hand.
Knowing that a high percentage of people favour the same side as their dominant hand when walking is not "insider trading".

The language one is a perfect example of why people bet. He thinks he is right (he could even be having the same doubt as you re fairness), you think you're right (I know, you are right in this case)

Having more knowledge about a subject is how people win bets. Think poker, sports betting or traders. Winners in all these aren't taking shots in the dark.

Is it unethical if a man offers to bet you that he can make the jack of spades jump out of the deck and squirt cider in your ear?

Is it unethical betting if your doctor offers to bet you that he knows someone with the same problem as you?

Son, do not accept this bet, because as sure as you stand there, you're going to wind up with an ear full of cider.

Instead counter-offer to either get your ass kicked by him or kick his ass and collect $200.

https://www.youtube.com/watch?v=SAIEIJHNFx4

of course! i'm struggling to think of one example where it's morally ok to use deception to enrich yourself.

i don't see a clear moral line here, one could probably make a convincing argument for either side.

yes, if you target someone and then lie to them or deceive them for personal profit, that would be an unethical act.

if you proclaim your ability to discern handedness, and your friend challenges your ability with a bet, it is not only morally acceptable, but is your moral imperative to take your friend's money for not believing in you.

I wouldn't use a phrase like, "I can tell handedness". I would just say, "I don't know that guy and have never seen him before but I'll put $500 on him being left handed". I might even encourage my opponent by saying, "there must be 10 righties for every lefty".

I have an extraordinary edge because most people don't realize that the dominant side can be detected from walking, at least when walking fast.

I'm not a gambler, at all, but I do play lots of poker at the casino. When it comes to betting outside of poker I would never make a bet unless I was confident I had a significant edge. Maybe this is why I would feel bad if my bet was a lock or nearly. It's like, I have too much of an edge. On the other hand most people responding think that's what betting is all about. Maybe a good rule of thumb is like what they say in poker, "if you can't spot the sucker..." Meaning, if you're not sure your opponents side is awful then you are almost certainly the one making the suckers bet and so decline.