Picture a rabbit at a lettuce leaf. The teeth touch, they take almost nothing, and then they come back and do it again. That is the whole word: a small, light, repeated touch of the teeth. Nothing about it is fast or , and no single bite matters much — what matters is that it keeps happening. Once you can see that picture, the other meanings are the same movement pointed at different things: at a person's ear, where it means love and play; at a fishing bait, where it means something came close but is not caught; at money or time, where a little keeps disappearing until a lot has gone.
From the word’s history
Dictionaries trace the verb to Late Middle English nebillen, 'to peck away at something', possibly from Middle Low German nibbelen, 'to eat in small bites, peck'; the origin is uncertain and may simply be imitative. Either way, the earliest recorded sense already carries the small repeated bite.
字典把這個動詞追到中古英語 nebillen（一點一點地啄、咬），可能來自中古低地德語 nibbelen（小口吃、啄）；來源並不確定，也有一說是模仿聲音而來。無論哪一條，最早記到的意思裡就已經有「小口、反覆」這個動作了。
Wiktionary etymology supplied in this packet (kaikki:3f6f69a190036ae4:713798317:67266): 'from Late Middle English nebillen… possibly from Middle Low German nibbelen… or imitative.'
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