(Zenmon no tora, koumon no ookami;
“A tiger at the front gate, a wolf at the rear”)
“Out of the frying pan, into the fire.” While you were driving away a tiger at the front gate, a wolf came in the rear gate. No sooner do you escape one misfortune than you run into another. Compounded troubles. A pincer attack. A situation that even a hero could not hold up against.
This kotowaza is simply a pair of noun phrases in the AのB pattern; in this case の is associative. (Which means that the English rendition “at” is not strictly a representative or equivalent to the Japanese grammar.) In each phrase 門 (mon) is “gate,” as in the gate of a traditional estate or compound; 前門 (zenmon) is the front gate and 後門 (koumon) is the rear. The animal at the front gate is 虎 (tora), “tiger,” and the animal at the rear is 狼 (ookami), a wolf.
In older script, the phrase would be written without a comma, which is after all a Western import. And this is a very old phrase; it seems to be derived from 「前門に虎を拒ぎ後門に狼を進む」, “Warding off a tiger at the front gate; advancing a wolf at the rear gate,” which itself descends from a work called (in Japanese) 評史 (hyoushi) by a Chinese writer called (again, in Japanese) 趙弼 (Chouhitsu).
(“Gakki no owari ga chikayoru to, dondon isogashiku natte oitsumeraremashita. Ronbun mo aru shi, gakusei-tachi no essei no saiten mo aru shi, zenmon no tora, koumon no ookami to iu joukyou desu.”)
[“I’m increasingly pressed for time as the end of the semester approaches. I have my own thesis to work on, and I have to grade the students’ essays. It’s like I’m caught between a tiger and a wolf.”]