means Not resolved, calm, or fixed in place — whether describing a restless mind, an unpaid bill, a turbulent sky, or an undecided matter.

from From 'settle,' which traces back to Old English 'setlan' — to seat, to place, to bring to rest — itself rooted in 'setl,' a seat or place to sit (a cousin of modern 'settee' and 'saddle'). Add the negating prefix 'un-' and you get the opposite of seated and steady: something that won't take its chair. The financial sense (an 'unsettled' account) grows naturally from the idea of accounts being 'settled' — literally brought to rest — once paid.