盗人の昼寝
(Nusubito no hirune; “A thief’s midday nap”)

Definition:

Even if an activity seems purposeless, it can still have a specific reason and purpose. Someone napping in the middle of the day may seem random, but for a thief the nap is important preparation for their “work” at night. Note that due to the connotations, this phrase is only used in a negative context, so it is not equivalent to the English “there’s a method to his madness.”

Breakdown:

This is another noun phrase made of two compound nouns connected with the associative particle の (no). The first is 盗人 (nusubito), made of “steal” plus “person” and meaning “thief.” The second is 昼寝 (hirune), made of “daytime / noon” plus “sleep” and meaning “daytime nap,” “siesta.” Good times.

Notes:

盗人 can also be pronounced nusutto, but can not be replaced with near-synonym 泥棒 (dorobou), “burglar / robber.” A longer, full-sentence version of the phrase also exists: 盗人の昼寝も当てがある (nusubito no hirune mo ate ga aru), “Even a thief’s midday nap has a purpose.”

The shorter form appears in the Edo and Osaka iroha karuta sets.

Example sentence:

「盗人の昼寝も当てがあるというから、学生のテスト中の振る舞いに注目しなければならない」

(“Nusubito no hirune mo ate ga aru to iu kara, gakusei no tesutochuu no furumai ni chuumoku shinakereba naranai.”)

[“Since, as they say, for a thief even a nap has a purpose, you have to pay close attention to how the students act during the test.”]