This kōan is an
idiom and the student is assumed to be aware of its
cultural context. If one does not know this context, the kōan cannot be understood from the traditional reference point.
The meal of consideration is a traditional meal of
rice. It was customary for monks to maintain
samadhi (the practice which produces complete meditation) while eating this meal, and so Zhaozhou is not asking whether the monk has eaten: he asks instead whether the monk was able to remain in samadhi throughout the meal. The monk affirms, and then realizes he has already received the teaching. This kōan is one of the 12 Gates taught in the
Kwan Um School of Zen.
The first principle of Zen is that there is no Zen. The monk was enlightened before he asked.
http://en.wikipedia.org/wiki/The_Gateless_Gate#Case_7:_Joshu_Washes_the_Bowl