I was watching a TV series when I came across this sentence. What does it mean? Where does this expression come from? The context was the following: A mother told her daughter that she got engaged, the girl's response was not very interested and she only said "wow" (in a sarcastic way). Then, the mother said: "Now, that response really put the 'luke' in lukewarm."
It is an informal or humorous expression. Explaining humor takes away all the fun but here goes. This is generally in the form of "He/she puts the A in AB." The AB needs to be a word where A is also a word or at least a syllable.
The usage here is the mother remarking on how unexcited the daughter is about the (should be) wonderful idea of getting married. Lukewarm describes the daughters response, meaning not so excited. The word 'luke' is not really a sensible word here but allows the mother to comment that the daughter does not seem as excited as she might be expected to be. I think its still funny.