Almanack

This is a calendar that includes information on important dates, as well as tide charts and other statistical information. (Today, we spell it without the K.) The most famous was Poor Richard's Almanack published by Founding Father and bifocal enthusiast Ben Franklin.

Capias

A warrant of arrest for someone. (Why don't they bring this word back? It sounds awesome.)

Dedimus Potestatem

A legal term that sounds like either a nasty rash or a poisonous mushroom, but is actually just a writ that gives power normally reserved for a judge to someone that's not a judge. Basically, it's a way to get judge-style superpowers.

Dice

Exactly the kind of dice that comes in a game of Monopoly. The ones of the time were more likely to be made of wood or ivory, but they'd be totally recognizable and usable for a modern person.

Ingross

Today, you'd say "engross"…if you said it at all. Ingrossing means to either copy something by hand, or to prepare the final version of an official document.

Nisi Prius

Any legal action tried in front of the King's Bench. It's basically an obsolete legal term from the 18th Century…although we think it sounds like the name of a top-secret super villain alliance from a Marvel comic.

Pamphlet

It means more or less the same thing then as it does today. The big difference is, back then, pamphlets were super-popular, being sold on street corners like hot dogs. A tax on pamphlets was significant.

Parchment

Paper made out of animal skin. Yeah, it's kinda gross, but history is an unremitting horror show at times.

Pence

The plural of penny. A British coin, it was worth around four farthings—which were real currency and not just something Harry Potter made up.

Playing Cards

Exactly what you're thinking of. The cards of the time were close enough to the modern version that you could easily use a deck without worrying too much. (Although you probably wouldn't want to, given that you could sell an 18th Century deck of cards for beaucoup bucks.)

Postea

A legal term meaning "afterwards." You'd have to assume they could just say "afterwards" then and leave it at that. But then how do you justify paying for law school?

Specifically, the postea appears on a nisi prius case with a record of what happened and who was there.

Pound Sterling

Another British coin. This was worth 20 shillings, or 4 crowns.

Shilling

Yet another British coin, this was worth approximately 12 pence. Notice how the denominations aren't exactly decimal.

Skin

In the context of the law, this just means something the colonists would write on. As in (generally) animal skin that hadn't been processed enough to be considered parchment or vellum.
Yes; the colonists were just writing on skin back then. No; they weren't serial killers.

Stamp

In the context of the Stamp Act, the stamp itself was sort of an official receipt. It looked like a modern rubber stamp without the rubber part. This would be dipped in ink and stamped on the document to prove the tax had been paid.

Vellum

High quality parchment. Still made out of animal skin…because life in the colonies was nasty.