The superlative form of costly, meaning the most expensive or requiring the greatest sacrifice or loss.
From Old English 'cost' (expense, price) combined with the superlative suffix '-iest'. The word 'cost' comes from Latin 'costare' (to stand at a price). The superlative form evolved in Middle English to compare multiple items by expense.
Interestingly, 'costliest' doesn't just mean most expensive in money—it can also mean most damaging or harmful, like 'the costliest mistake.' This shows how our language treats financial expense and human sacrifice as fundamentally similar kinds of loss.
Complete word intelligence in one call. Free tier — 50 lookups/day.