Those who ask this question must either forget that the Book of Mormon (and for that matter, the Bible) is a translation from another language, or perhaps they don't realize that a translator uses the words in his vocabulary which he feels will best portray the meaning of the original writer's or speaker's thoughts to current readers.
Strictly speaking, no words in the Book of Mormon are Jacob's words, or the words of any other prophet of that period. "Adieu" was probably the best equivalent of Jacob's words of which Joseph Smith was aware. So, whether it is "adieu" or "constraineth" or "abomination," they are modern translations of ancient words.
It is interesting to note that Daniel LI. Ludlow, an LDS Church scholar, has found a Hebrew word that means the same or nearly the same as Adieu. It is "Lehitra'ot" (A Companion to Your Study of the Book of Mormon, p.163). Both words mean "I commend you to God."
A Biblical parallel detractors should consider is the use of the word "compass" in the Old and New Testament. Most scholars credit Chinese navigators with the invention of the compass around 1000 AD (World Book Encvclopedia, Vol.4, 1972, p. 735), which is nearly 1,000 years after the latest reference to a compass in Acts 28:13. Certainly this easily explained problem of word choices would not shake anyone's faith in the Bible. Nor should the word choice of a 19th-Century prophet shake anyone's faith in the Book of Mormon.
Another example of an equivalent modern word in the Bible is the use of "Easter" in the King James version of Acts 12:4. Most modern translations reject this modern word in favor of the ancient term "Passover."
The author once asked a Church of Christ minister about the words compass and Easter in the Bible. His answer was, "Obviously there was a mistake in translation." Because of translation issues and errors, Joseph Smith wrote in our eighth Article of Faith: "We believe the Bible to be the word of God as far as it is translated correctly; we also believe the Book of Mormon to be the word of God."