Learn English – Does “plain Jane” really mean ugly Jane? If so, why

When the English settlers landed in the New World, they didn't have a word for maize. Maize is a New World crop which was unknown in Europe. The word "maize" was originally Spanish, and comes from the word "mahiz" in the Arawak language of Haiti, and in the early 1600s it was not yet a common word in England. The settlers called it "Indian corn", which soon got shortened to just "corn".

EDIT: In the comments, some people are questioning whether "Indian corn" and "maize" refer to the same thing. They certainly don't today; in the U.S. Indian corn usually means ears of maize with multicolored kernels which are grown primarily for decoration. However, both terms were used and appear to have been treated as synonyms in the U.S. during and before the 18th century. From a section of An Universal History (London, 1763) discussing New England:

We have already observed that the country is fruitful in all kinds of esculent plants, pulse, and corn; but Indian corn, or maiz, which the natives call Weachin, is the most cultivated, and was alone known here on the first arrival of the Europeans. The following is the account of it communicated to the royal society by Mr. Winstrop ... "The ear is a span long, composed of eight or more rows of grain, according to the quality of the soil, and about thirty grains in each row, so that each ear at a medium produces about two hundred and forty grains, which is an astonishing increase. It is of various colours, red, white, yellow, black, green, &c, and the diversity frequently appears not only in the same field, but in the very same ear of corn, though yellow and white be the most common. ... It is observable, that the maize dwindles the farther you advance to the northward ... sufficiently evince the Indian corn to be a native of the more southern latitudes ... "

"Consound it" means the same thing as "confound it".

My guess is that the interjection "confound it" was thought to be too strong in Hannibal, Missouri, at the time of Mark Twain's childhood. So people changed the pronunciation slightly to avoid using "bad words". (I don't see anything objectionable in "confound it", but maybe it was perceived as a euphemism for "damn it" that was somehow too close to the original.)

When Mark Twain was editing his original draft of Huckleberry Finn, he changed at least one instance of "consound it" to "confound it". See this link. Tom Sawyer contains both "confound it" and "consound it".