Grammatically the use of the third person singular meaneth disagrees with the plural subject, the words, although in the original Book of Mormon text such usage is fairly common (that is, the inflectional ending -(e)th often occurred with plural subjects). For the verb mean, most third person plural subjects take the base form of the verb in the original text rather than the -(e)th inflectional ending:
Note that in 1 Nephi 22:1 the original mean was accidentally replaced by meaneth in the 1830 edition (see the discussion under that passage). The problem with these wh-interrogative clauses is that the sentence-initial what tends to be misinterpreted as the subject. This kind of misinterpretation would also explain why subsequent editors have missed editing meaneth to mean here in Mosiah 12:20 as well as in 1 Nephi 22:1. Indeed, one could argue that meaneth in Mosiah 12:20 may be an error for mean, given that the change of mean to meaneth actually occurred in 1 Nephi 22:1. But it should be noted that in the earliest text there is one more example involving the verb mean where the finite verb in the earliest text has the -(e)th ending:
Thus in these wh-interrogative clauses the original text allows for plural subjects to take verbs with either the standard zero ending or the nonstandard -(e)th. We therefore follow the earliest textual sources in each case, which means that in Mosiah 12:20 the verb form meaneth will be retained and not interpreted as an early scribal error for mean.
The King James Bible systematically distinguishes between the third person singular -(e)th ending and the plural zero ending. Thus in the King James text we have only the grammatically correct mean with third person plural subjects in what-initial clauses:
Summary: Maintain the nonstandard use of the third person singular form meaneth in Mosiah 12:20.