If you are interested in word origins, the Oxford English Dictionary is truly the best way to go. The Oxford English Dictionary, or O.E.D. for short, is over 30 volumes in it's unedited form, in part because the original authors painstakingly researched the root of every word entry, when possible. The unedited form of the O.E.D. quotes the first sentence to ever use each word entry in writing (if written records go back that far) and the author of the sentence. I have the Concise Oxford Dictionary here in my office, which is only one volume but includes some information on word origins. Under "neap", on page 909, the origin is said to be the Old English word "nepflod", which is very similar to the Old English word "flod", which meant flood. The exact origin of "nepflod" itself is unknown, but the word "flod" is Germanic. Many of the oldest words in the English language are Germanic, which means they are derived from the language of the Germans who lived in the 5th century--the language of the Anglo-Saxons--which is very different from German language as it is spoken today. (Similarly, we would have a hard time understanding and reading Old English as it was spoken and written back in the 10th-16th centuries.) I suggest you go to the public library and look up the work "neap" in the full version of the Oxford English Dictionary (if they have it) and see if there is more information. It could be that the word is so old that there is no written record of its origin. This is true for many Old English words. When do you think the first book was printed?