Gin up (or ginned up) means enliven, excite or enthuse. Its probable derivation is from the 1800's British slang term "ginger up," which referred to the practice of putting ginger up a horse's butt to make him spirited and prance with a high tail, for purposes of show or sale. The other term for this practice is the verb "feague." This is confirmed both by the online Phrase Finder from the UK and the OED. The Phrase Finder reports its use in the US prior to 1895.