Scott Adams (of Dilbert fame) posted about a term he learned from his in-laws: double-bagging. His story about what it means in this context is cute, but you can read about it at his blog. What I find interesting is the use of an ordinary term typically referring to using two bags instead of one for groceries (as reinforcement) for the unusual situation involving the dog Millie. Like most linguists, I was required to study some historical linguistics and socio-linguistics involving language change. My memory is fuzzy, but I recall vaguely that there are models of neologism formation that account for the various ways an existing term gets transferred to a new domain.
What's interesting to me about "double-bagging" is that the salient part of the term is the instrument, not the action, because the only thing the two uses share is the need for two bags. The way in which the two bags get used in each situation is, in fact, quite different. So, rather than foregrounding the similarity of the situations (the way metaphor might), this is a case where two unrelated situations happen to share an instrument in common and it is the instrument which forms the neologism. I wonder if instruments in general lend themselves to this kind of linguistic process? Are there other cases where two dissimilar situations share an instrument (used in different ways) but have the instrument form a neologism?