A riding crop (or hunting crop) is a short stick which usually has a piece of leather on the end. It is carried by horse-riders and used as a whip. When something is in the back of your mind it is in your thoughts, but is not the main thing you are thinking about. For example, you may be having a nice drive in your car, enjoying the scenery, feeling happy... but in the back of your mind is the thought of the cost of the fuel (petrol/diesel/gas). Lickered up is a phonetic/slang spelling of liquored up (which is itself slang/very informal). It means you have drunk a lot of alcohol (liquor). I believe it is common in North America (the expression, not getting drunk!!!). It's not so common here in England but we use it sometimes (especially if we've been watching |American movies!). Here in England it's probably the getting drunk that's common, not the phrase! If you want the origin of the word try the dictionary under 'LIQUOR'.