Big fish devour small fish - the strong prey on the weak.
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噬 is a literary/classical character. In modern speech, 吃 or 咬 are used instead. 噬 appears in formal or poetic writing, and in compound words like 吞噬 (to engulf) and 反噬 (to backfire).
Left mouth radical - pictograph of an open mouth, a small square. 噬 means to bite or devour, so the mouth is the agent. Same family: 吃 eat, 咬 bite, 吞 swallow, 啃 gnaw. The literary 噬 carries a menacing flavour - predators biting, not people eating.
Right 筮 supplies the sound shì with no shift - a tidy phonetic. 筮 depicts a sorcerer (巫) drawing yarrow stalks (竹) for divination. The literary, archaic feel of 筮 rubs off on 噬: not an everyday bite but a classical-register devour, as in 噬脐莫及.
