fish biting; fish striking; fish taking bait
Refers to the action of a fish biting or taking the bait, often used in fishing contexts.
今日は魚の食いが悪い。
The fish aren't biting well today.
魚の食いがいい時間帯を狙う。
Aim for the time when the fish are biting well.
Compound of 魚 (fish) + の (possessive particle) + 食い (biting, eating). The exact historical derivation is straightforward; the expression is transparently formed from its components.