Take It Or Leave It

Usually a phrase which means: Either accept the item in question as it is or do not. Either way, do not complain about it. This is not a phrase of love.

I told him I'd already written code to do the same thing he was doing but he didn't seem interested. He was clearly suffering from NotInventedHere (NIH) syndrome.

... or was he?

Just because you have worked out the logic for one little thing that happens to have a recognizable name doesn't mean that every other programmer in the world has an obligation to reuse your code. If a developer can tell by the way you talk that you don't understand his problem or care to learn about it then you will surely find a lack of interest on his part. If you call this NIH then you are just fooling yourself.