(Nusubito no hirune; “A thief’s midday nap”)
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.”
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.
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.
(“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.”]