同じ釜の飯を食う
(Onaji kama no meshi wo kuu; “To eat rice from the same pot”)

Definition:

Leading the same lifestyle together and working together; going through good times and bad together; being extremely close friends. An intimate, familial sense of closeness that comes from shared experiences and activities.

Breakdown:

We begin with adjective 同じ (onaji), “the same,” attached to and modifying 釜 (kama), a traditional cast-iron cook-pot. This is followed by the noun 飯 (meshi), “boiled rice,” or by extension “meal,” which is connected to the pot by the associative particle の (no). The rice/meal is marked by the particle を (wo) as the object of the verb 食う (kuu), “to eat,” which appears in sentence-final form.

Notes:

同じ may be replaced, more or less synonymously, with 一つ (hitotsu), “a single.” However, replacing the particle の with で (de), “at,” is considered an error.

Example sentence:

同じ釜の飯を食った幼馴染に裏切られたと思うだけで、悔しさの極みだ。

(Onaji kama no meshi wo kutta osananajimi ni uragirareta to omou dake de, kuyashisa no kiwami da.)

[To even think that you might have been betrayed by one of your oldest, closest friends is the most painful thing in the world.]