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Beside dictionaries noone knows the difference. In fact there are cases when there's no difference.
https://dictionary.cambridge.org/us/grammar/british-grammar/across-over-or-through
> Especially when we use them as adverbs, over can mean the same as across:
> We walked over to the shop. (or We walked across to the shop – the shop is on the other side of the road)
> I was going across to say hello when I realised that I couldn’t remember his name. (or I was going over to say … meaning ‘to the other side of the street or room’)
Come on give us some etymologies. Or that's too beneath you?
Fugging Hungarians, srsly.

PS. noone is not a real word, the correct term would be nobody.