マンネリ

A curious word this and a good example of those false friends known collectively as 和製英語（わせいえいご）: English words that have entered the Japanese language but in the process somehow shed their original meaning and come to mean something quite new and often, strange.

マンネリ (manneri) is short for マンネリズム (mannerizumu) which of course originally meant a mannerism but has come to mean getting stuck in a rut, the same old same old routine type-o-thing. There are some good examples of its usage up here. Unfortunately, I learnt this word when my girlfriend told me that we were マンネリカップル i.e. our relationship has become stuck in a humdrum groove with no spark or excitement... which gave me pause for thought. I shan't forget that word in a hurry. Now to go out and buy some leather goods...