yappy: A slang term used as an equivalent of "foolish" which is to be preferred.

yes: Discard such vulgarisms as yeh and yep and pronounce as a single syllable, and not with affectation, as, sometimes in England ya-as, or with a Yankee drawl ye-es. Avoid, too, the objectionable habit of using this word as the sole response in conversation; a habit which is indeed fatally destructive of conversation, which should partake more or less of an interchange of ideas. "Yes! she would reply encouragingly ... and 'yes!' conclusively, like an incarnation of stupidity dealing in monosyllables." (Meredith, "Beauchamp's Career," vol. iii. ch. 10, p. 185.) Also, when speaking in English do not inject the German "Ja!" when you wish to signify assent. This practice is rapidly gaining ground among the middle class.

Yid: A Jew: an appellation common among the vulgar and therefore one to be avoided.

you even when used in relation to one person, is still grammatically plural, always requiring the plural verb; as, "You were fortunate," not "You was fortunate"; "If you were to curse you would sin," not "If you was to curse," etc.

you and I, you or I: Phrases in which the objective pronoun me and the first personal pronoun I are often confused; as, "This will not do for you and I," instead of "This will not do for you and me.'" The rule is very simple, viz.: use I or me in such connection just as if the words "you and" or "you or" were omitted. "They were not citizens as (you and) I"; "He is not so tall as (you or) I."

you don't say? Compare is that so?

your's truly: An incorrect form, yours being a possessive pronoun does not need the sign of the possessive after it.