This week, it looks like we're gonna be working off of a general theme of "interest."
Yesterday's 興味津々 leads us nicely into the idea of today's Language Trivia, albeit with a slightly different twist.
up to the neck
This one is useful when you want to talk about an overwhelming interest that borders on devotion, ESPECIALLY when that interest is in a person of the opposite sex.
Used often (even, defined often) with 夢中, which I'm pretty sure we've covered before.
Saying 「誰か」or 「何かに夢中」 is to say that you are "infatuated" and gets employed when translating what we label, in English, "a crush."
首っ丈, on the other hand, also used with に, does carry the weight of "devotion," so it might be better translated as "to only have eyes for..."
I'm willing to bet you'll get major points when you use it on your significant other, and, if you do have a significant other, try using it as a response to those people who ask about your モテモテ status, like so:
Random Japanese person: ブレットさんはモテモテだろう。
Brett, you must be fighting the ladies off with a stick, right?
Actually... I only have eyes for my girl. (so... yes. It's a heavy stick.)
(Artist's rendering of Brett. Pretty faithful.)