目に入れても痛くない
(Me ni irete mo itakunai; “Doesn’t hurt even if it gets in your eye”)

Definition:

To adore someone, especially a child or grandchild, beyond all reason. To dote on someone. Generally reserved for feelings toward “cute” things such as small children or, occasionally, pets.

Breakdown:

We begin with the noun 目 (me), “eye,” marked by the particle に (ni) as the target of the motion of the verb 入れる (ireru), “to insert,” “to put [something] in[to something].” This verb appears in conjunctive form and is followed by the emphatic particle も (mo), “even [if].” Finally we have the adjective 痛い (itai), “painful,” in conjunctive form and taking the negative adjectival suffix ない (nai), in sentence-final form.

Notes:

The adjective may be replaced, in some versions, with えずい (ezui), “unpleasant” – although this seems to be rare. Other versions make the placement even more explicit by making it 目の中に… (me no naka ni…).

While none of my written sources say this, and some of the usage examples I found explicitly contradict it, I’ve been told in person that this phrase is only really used regarding members of one’s own household or family – that the person doing the doting may be the speaker, or their parent or grandparent, and the object of the doting may be a child or grandchild, but that in any case it’s only used when everybody involved is related to the speaker in some way or other. You may take this assertion into advisement if you wish.

Example sentence:

「公園で娘さんを見守ってる山野さんの顔を見た瞬間、あ、目に入れても痛くないってこういうことなのかなって思った」

(“Kouen de musume-san wo mimamotteru Yamano-san no kao wo mita shunkan, a, me ni irete mo itakunai tte kou iu koto na no ka na tte omotta.”)

[“The instant I saw Mr. Yamano’s face while he was watching over his daughter at the park, I thought, Oh, is this what it means to be the apple of someone’s eye?“]