What's a concertina word?
A concertina word, word Z, is a word in English which can be formed from splicing together two other English words, word X and word Y, such that the letters of word X form the odd numbered letters of word Z, and word Y, the even ones.
It might be simplest to explain with a simple example:
The words FRIENDS is a concertina word because it can be formed from the words:
1234567 F I N S and R E D
While the prize ('what prize' I hear you ask) goes to the longest word, bonus points ('what bonus points') are on offer for interesting answers - for instance, where words x, y, and z make a plausible phrase, or where the subwords resonate with the meaning of the superword.
Rosie F advises that this sort of construction is actually called an alternade q.f. https://en.wikipedia.org/wiki/Alternade (although I like my term better).