* Showing that the word is being used in a specific dialect or regional language. In this case, "labboard" refers to the left side of a boat, a term used by sailors. Twain was known for capturing the distinctive language of different regions and social groups.
* Drawing attention to the word's unusualness or archaism. "Labboard" is an older, less common term compared to the more familiar "port". Twain might be using the quotation marks to highlight the historical or unusual usage of the word, possibly for humorous effect or to emphasize the character's background or personality.
It's important to remember that Twain often used language creatively, and his specific intentions with quotation marks might vary depending on the context of the sentence and the overall narrative.