you just don’t get it → See under GET IT, def. 2.
you know You are aware, you see, do you remember, as in She’s very lonely, you know, so do go and visit, or You know, this exhibit ends tomorrow, or You know that black dog our neighbors had? She was run over a year ago. This phrase is also quite often a conversational filler, equivalent to “um” and occasionally repeated over and over (as in It’s a fine day for, you know, the beach, and, you know, we could leave now); this usage is more oral than written, and many consider it deplorable. [Late 1500s]
you know something? Also, you know what? Listen to what I’m going to tell you, as in You know something? He’s always hated spicy food, or You know what? They’re not getting married after all. Both these colloquial expressions are shortenings (Of Do you know something? or Do you know what?) and are used to emphasize the following statement or to introduce a surprising fact or comment. The first dates from the mid-1900s. The variant, from the late 1800s, should not be confused with WHAT DO YOU KNOW or YOU KNOW.
you name it Everything one can think of, as in We’ve got a crib, highchair, diapers—you name it. [Colloquial; mid-1900s]
you never can tell Also, you never know. Perhaps, possibly, one can’t be certain, as in You never can tell, it might turn into a beautiful day, or You may yet win the lottery—you never know. The first term uses tell in the sense of “discern,” a usage dating from the late 1300s; the variant dates from the mid-1800s.
young at heart Having a youthful outlook, especially in spite of one’s age. For example, She loves carnivals and fairs; she’s a grandmother but she’s young at heart.
you’re telling me I’m well aware of that, as in She’s a terrific dancer.—You’re telling me! I taught her how, or You’re telling me, the prices are sky-high here. [Early 1900s]
you’re welcome Also, don’t mention it; not at all. No thanks are needed, I was glad to do it. For example, Thanks for picking me up.—You’re welcome, or I appreciate what you did for Mother.—Don’t mention it, or I loved your e-mail—not at all. These phrases are polite formulas for responding to thanks. The first dates from about 1900; the first variant was first recorded in 1841; the second in 1846. For synonyms, see FORGET IT; NO PROBLEM, def. 2. Also see WELCOME TO.
your guess is as good as mine I don’t know any more than you do, as in As for when he’ll arrive, your guess is as good as mine. [1920s]
yours truly 1. A closing formula for a letter, as in It was signed “Yours truly, Mary Smith.” [Late 1700s] 2. I, me, myself, as in Jane sends her love, as does yours truly. [Colloquial; mid-1800s]
you said it → See YOU CAN SAY THAT AGAIN.
you scratch my back and I’ll scratch yours → See SCRATCH SOMEONE’S BACK.
you’ve lost me I can’t follow what you’re saying, I’m puzzled or bewildered. For example, Please explain it again; you’ve lost me. This turn of phrase transfers losing one’s way to discourse. [c. 1960]
z → See FROM SOUP TO NUTS (A TO Z).
zap out Interrupt or delete unwanted parts of a television program or video, as in We’ve taped the show and now we can zap out all the commercials. [c. 1980]
zero in on 1. Aim precisely at a target, as in They zeroed in on the last snipers. [c. 1940] 2. Direct one’s attention to, concentrate or focus on, as in We must zero in on the exact combination of ingredients, or The whole class zeroed in on the new assignment. This usage transfers aiming a firearm to directing one’s attention. [Mid-1900s] 3. Converge on, close in on, as in The children zeroed in on the electric train display. [Mid-1900s]
zone out Stop paying attention, dissociate oneself from a situation. Also, engage in a mindless activity. For example, When Felicia starts talking about her ailments and her friends’ ailments, I totally zone out. This idiom also occurs in the passive, be zoned out. It originally alluded to narcotic intoxication and then was broadened to other kinds of dissociation. For a near synonym, see TUNE OUT, def. 2. [Slang; second half of 1900s]
zoo → See IT’S A ZOO.
zoom in on 1. Obtain a closeup view of the subject with a camera, as in The TV people zoomed in on the Olympic gold medalist. [Mid-1900s] 2. Focus on, examine closely, as in The moderator got the panelists to zoom in on the health-care issue. [Second half of 1900s]
The American Heritage Dictionary of Idioms Page 134