Liquid comes from Latin liquidus meaning 'flowing, fluid', from liquēre ('to be liquid'). The financial sense — liquid assets — treats money as water. Liquidate acquired its killing sense from Soviet euphemism.
Having a consistency like that of water or oil; flowing freely; not fixed or stable.
From Latin liquidus meaning 'fluid, flowing, moist', from liquēre meaning 'to be fluid, to be liquid'. The Latin liquēre derives from PIE *wleykʷ- meaning 'to flow, to be moist'. The financial sense — liquid assets — emerged in the 19th century: money that flows as easily as water, convertible without friction. Liquor shares the same root: it is simply 'the liquid'. Liquidate originally meant 'to make clear, to settle accounts' — to make debts
Liquidate started as an accounting term meaning 'to make debts clear and settled' — to make them flow away like water. The sinister meaning ('to kill, to destroy') came from Soviet Russian usage in the 1920s, where likvidirovat' was a bureaucratic euphemism for execution. English borrowed the euphemism. Now the word that once meant 'to clear an account' means 'to eliminate a person'.