Words that I have come across in reading history also seemed to have changed, or at the least are used with double meanings. I will use two words to illustrate my point: peculiar, and likely.
Most students of slavery are familiar with these terms, especially peculiar. The term has been used endlessly to describe the practice of slavery itself. John C. Calhoun seemed to have first used the phrase "peculiar institution" in a speech on February 2, 1837, while arguing against the reception of antislavery petitions in Congress. To most people today peculiar means odd, strange, weird, unusual, but I personally think that Calhoun mean peculiar in a much different way. Webster's gives the following definition: "characteristic of only one person, group, or thing : DISTINCTIVE." Only the next listed definition connotes strangeness. Here is Calhoun's quote: "The peculiar institution of the South - that, on the maintenance of which the very existence of the slave holding States depends, is pronounced to be be sinful and odious in the sight of God and man; and this with a systematic design of rendering us hateful in the eyes of the world - with a view to a general crusade against us and our institutions." From this I think that he meant distinctive or characteristic to the South, not strange or unusual, especially since he used "us and our" for clarification.
Here is an example from history that will give the same use of peculiar that I personally believe Calhoun meant. This is from Mississippi's statement of justification for secession written in 1861. "Our position is thoroughly identified with the institution of slavery - the greatest material interest in the world. Its labor supplies the product which constitutes by far the largest and most important portions of the commerce of the earth. The products are peculiar to the climate verging on tropical regions, and by an imperious law of nature, none but the black race can bear exposure to the tropical sun. These products have become necessities of the world, a blow at slavery is a blow at commerce and civilization.
The other word, likely, seems to have been used both similarly to how we use it today and also quite differently. Likely to me today means probably. Webster's says, "having a high probability of occurring or being true." I often say something similar to, "It is likely to rain today." But after reading numerous slave sales and runaway advertisements, people of the past had a different meaning for likely. For example, "To be SOLD, for want of employment, an exceedingly likely NEGRO GIRL, aged sixteen," reads one such advertisement. Another says, "Twenty Dollars Reward. Ran Away from he subscriber, on the 1st of May, Patty, a likely Negro wench, about twenty years of age...." In these instances likely is to meant to follow Webster's current fourth and fifth listed definitions; "4 : PROMISING, 5 : ATTRACTIVE.
I did find an historical example that uses likely in both the modern and historical meanings. "Run away from Sachervall Wood, living in Philadelphia, on 25th of October last: One James Anderson, an apprentice lad between 18 and 19 years of age, about 5 feet 8 inches high, a tailor by trade, and had better than 2 years to serve, he has a smooth face and fair complexion, and wears his own hair, and is pretty likely...He has a Mother and a Brother living in East town in the Jerseys, its likely he may be gone that way.
I have been searching for quite a while for a mid-19th century dictionary to help me when I run into instances such as this. I doubt that I will find one other than in an archive or special collections, but it would be helpful to have for such instances.
So, if you run into words while reading primary sources that don't sound quite right, don't be too concerned, it is quite likely that these peculiar words have changed over time too. Happy reading!