I like this kotowaza so much, I think I’ll post it next week too!

馬鹿の一つ覚え
(Baka no hitotsu oboe; “The one thing a fool remembers”)

Definition:

“He that knows little often repeats it.” A foolish person clings to one thing, and proudly repeats it (says the same thing over and over, does the same action over and over) in every situation, regardless of whether it’s appropriate or not. Used to poke fun at people who commit such repetition, especially if they seem overly enamored of the concept they’ve glommed onto.

Breakdown:

馬鹿 (baka) is a fool, an idiot, a simple-minded or unthinking person. Note that the characters literally mean “horse deer,” and it’s unclear how the combination came to have this meaning – you can read about some extant theories here (in Japanese). Next comes the associative particle の (no), in this case taking on a possessive function that refers to everything that follows. First we have the number 一つ (hitotsu), “one,” “single,” and the nominalized form of the verb 覚える (oboeru), “to remember.”

Notes:

Nothing special about this one that I can find. It doesn’t even seem to be that old, with attestation since 1951.

Example sentence:

「馬鹿の一つ覚えみたいにどんな現象であっても原因が量子力学だろうと、この科学者は常に言っている」

(“Baka no hitotsu oboe mitai ni donna genshou de atte mo gen’in ga ryoushirikigaku darou to, kono kagakusha wa tsune ni itteiru.”)

[“Like the one thought in a fool’s mind, this scientist was always talking about how this, that, or the other phenomenon was probably based in quantum mechanics.”]