What is a milk neck? Think about where milk comes from. Now, think of what looks like a little neck on the thing that milk comes from. What is that thing? It's a nipple.

The reading for this one is an exception, at least for the 乳 part. It's read as ち, and 首 is read as くび. You can remember the reading of nipple by thinking of a very special "nipple" you have on your face. A bee has burrowed into your cheek, and it squirts out milk from its butt, which sticks out of your cheek. You can't get it out, so you've decided to call this little cheek bee nipple your cheek bee (ちくび). Buzz buzz *milk*