壁に耳あり障子に目あり
(Kabe ni mimi ari shouji ni me ari;
“The walls have ears, the doors have eyes”)

Definition:

If you’re talking about something that you want to keep secret for any reason, be careful. Keep in mind how much of old Japanese estate construction was based on thin walls, screens, curtains, and partitions. It was practically a literary pastime to find ways to peek at people – for aristocratic men to try to catch glimpses of ladies they were courting, for example. So there could always be people peering through hedges, trying to get an angle to look past a carelessly-placed partition, poking holes in paper screens to peek through, or simply listening to anything they could overhear. And that’s not even counting the ubiquitous swarms of servants (themselves generally lower-ranked aristocrats receiving patronage from their richer, better-connected fellows rather than low-caste workers like you’d see in the West).

Why would Japanese culture develop a rich tradition of indirectness, of discretion and tactical silences, of talking all around a matter without ever alluding to it directly? Because someone’s always listening.

Breakdown:

Another repeated structure. Note that each half of the phrase functions as a complete sentence on its own.

壁 (kabe) is a wall, marked with に (ni), here a positional marker. 耳 (mimi) is “ear(s),” and the verb あり (ari) is the “to be” verb in its old-fashioned sentence-final form. Although in modern grammar the verb has essentially been regularized as ある (aru), in the Heian era it was essentially its own verb class, and vestiges of that heritage remain in the relatively common usage of あり, which would otherwise be an incomplete form. So you have “In the wall, there are ears.”

In the second half of the phrase, the particle and verb remain the same, and the two nouns change: the first is 障子 (shouji), a standing paper screen or sliding paper door. The second is 目 (me, pronounced like “meh,”) “eye(s).”

Notes:

Apparently there are quite a few variations on the basic theme here. Some substitute in rocks (石, ishi) or hedges (垣, kaki); some add a mouth (口, kuchi) for what has been overheard to be passed on.

In keeping with the grammatical completeness of each half of the kotowaza, either half (especially the first half) can also be used on its own, as shown below.

Example sentence:

「あれ、持って来た？」　「後で話す。この部屋、壁に耳あり」

(“Are, motte kita?” “Ato de hanasu. Kono heya, kabe ni mimi ari.”)

[“Did you bring it?” “We’ll talk later. The walls have ears in here.”]

Bonus Media!

Source. The joke is that the name “Mary” (メアリー) sounds like 目あり. So there’s an ear at the wall and Mary at the door. Being creepy. Again.