物言えば唇寒し秋の風
(Mono ieba kuchibiru samushi aki no kaze;
“When you say something, then your lips will feel the chill of an autumn wind”)
Definition:
Being nasty will only make you feel bad later; shooting off your mouth invites disaster. Needlessly talking about other people’s (perceived) shortcomings and boasting about your own (perceived) virtues only invites ill-will and misfortune, so it’s better to be circumspect. “Silence is golden.” From the image of opening your mouth (for malicious gossip, etc.) and feeling an unpleasant chill as the autumn breeze brushes your lips.
Breakdown:
We begin with noun 物 (mono), “thing,” compounded with verb 言う (iu), “to say.” The latter is in perfective form, with conditional suffix ば (ba), “when(ever).” This dependent clause is followed by noun 唇 (kuchibiru), “lip(s),” with adjective 寒し (samushi), “cold,” in conclusive form. This is followed by what I parse as a separate comment: a noun phrase comprising 秋 (aki), “autumn,” and 風 (kaze), “wind,” joined by associative particle の (no). One might imagine particles (such as an を between 物 and 言えば) or other additions, but these are elided.
Notes:
Careful readers will already know why the grammatical structure is a bit unusual for a kotowaza: it’s because this is also poetry. This “saying” is a 5-7-5-syllable hokku from the poetic collection『芭蕉庵小文庫』 (Basho-an kobunko) by, well, famed poet Matsuo Bashō.
That said, it’s acceptable to shorten the saying to a pithier form such as 物言えば唇寒し.
The wisdom of keeping one’s mouth shut is not a new topic.
Example sentence:
「ママ、ママ、テストで百点満点取った！」「良くできたわね！嬉しいね」「でも、はるちゃんは」「そんなの言わなくていいわよ、物言えば唇寒し」「え？普通にあったかいけど」
(“Mama, mama, tesuto de hyakuten manten totta!” “Yoku dekita wa ne! Ureshii ne.” “Demo, Haru-chan wa” “Sonna no iwanakute ii wa yo, mono ieba kuchibiru samushi.” “E? Futsuu ni attakai kedo.”)
[“Mama, mama, I got a perfect hundred on the test!”
“Well done! I’m so happy!”
“But Haru got a—”
“No need for that. Say too much and it chills the mouth.”
“Uh? But I’m as warm as usual.”]
landofnudotcom 
Thanks for another interesting post!
One question, why are you parsing “秋の風” as a separate comment? Can’t 寒し be directly modifying 秋の風? That seems to make more sense, but grammatically I’m not sure about how the ~し conjugation works here.
Classical Japanese is a tricky beast with a complicated history, but in my understanding 寒し is going to be the conclusive form (終止形, which I used to call “sentence-final”). It does have some uses other than finishing a thought, but if you specifically wanted to use 寒 to modify 秋の風, then most likely you’d want the 連体形 (prenominal) form, which is 寒き.
For that matter, if you read this with 寒し somehow modifying 秋の風, then you end up with not one but two fragmentary noun phrases, with 物言えば唇 standing on its own. “If you speak, it chills the lips” feels more right to me than “If you speak, then lips.” ;p It’s also a bit unusual to have a break between two thoughts or phrases in the middle of the seven-mora block like that.
I’m open to alternate interpretations, which is why I said that the reading above is how I personally parse the lines, but I’m pretty confident in this one.