Question: I've been told that the use of "gender" to mean "sex," as in "a person of the masculine gender," is incorrect and that the word should only be used in its grammatical sense. Is this true?
Answer: Some people think so, regarding the "sex" sense of "gender" as a recent development. In fact, though, this sense of "gender" is just as old as its unstigmatized grammatical sense. (The grammatical sense relates to the classification of words into masculine and feminine subclasses in certain languages. In French, for example, the word "crayon," meaning "pencil," is a noun of the masculine gender and requires the masculine definite article "le," while the word "plume," meaning "pen," is feminine and requires the feminine definite article "la.")
Written evidence for both the grammatical sense and the "sex" sense goes back to the 14th century. Both senses are treated as standard English in the dictionaries of Samuel Johnson (1755) and Noah Webster (1828). The evidence suggests that the use of the "sex" sense during the 19th century was, if not common, at least unremarkable.
Around the turn of the 20th century, though, dictionaries began to apply labels like "obsolete" and "jocular" to this sense, indicating that it had fallen into relative disuse by then. Not long afterward, it began to attract the unfavorable attention of usage commentators, who have been critical of its use since at least the 1920s.
Despite the criticism, however, the "sex" sense of "gender" has become increasingly common in recent decades, and the restrictive labels formerly found in dictionaries have long since been dropped. Its revival seems to be partly attributable to the increased public attention now being given to issues involving men and women, as well as the increased use of the word "sex" in senses having more risque connotations.
In any case, there is no denying that the "sex" sense of "gender" is now more common than it has ever been. It is not an error, but there are still many people who dislike it, as you've seen for yourself, and you should not expect the criticism (much less the sense) to disappear anytime soon.
Question: I would like to know what the meaning is for the following: "Never look a gift horse in the mouth."
Answer: Dating back to before the 16th century, this old proverb admonishes the beneficiary of a gift to abstain from appearing ungrateful by questioning or checking the quality or value of the gift. An understanding of the expression requires the knowledge that a horse's teeth grow longer with age - the longer the teeth, the older the horse. By looking in its mouth, the recipient of the horse could tell the age, and thus the value, of his or her gift. This is, by the way, also the origin of the expression "long in the tooth," meaning "old."
Question: One can say or write "I know what it is" but not "I know what it's." Since the contraction "it's" equals "it is" (doesn't it?), why the difference?
Answer: To answer your parenthetical question first - yes, the contraction "it's" is equivalent to "it is." The form without an apostrophe, "its," is the possessive of the pronoun "it," as in "The cat licked its paw." Many people find the missing apostrophe confusing, since an apostrophe normally precedes the possessive suffix "-s" ("George's car," "the cat's tail"). Possessive pronouns, however, have no apostrophe ("yours," "ours," "hers," "theirs," "its").
But that's not what you asked. Your question is a good one, and it has no easy answer. This is ultimately a matter of idiom, which is a fancy way of saying that a sentence like "I know what it's" is not possible in English because, simply, it doesn't sound right.
Looking at the question a bit more analytically, we can note that the usual effect of placing a verb at the end of a sentence, as in "I know what it is," is to give it special emphasis. Reducing "it is" to "it's" in this case entirely eliminates that emphasis, making the verb so inconspicuous as to be practically invisible. It's likely that "I know what it's" sounds so peculiar because of the way it de-emphasizes the verb "is." In any case, it does sound peculiar, and for that reason it is something that a native speaker of English would never say.