Superlative form of tall; the greatest height or the one with the most height.
From Old English 'getæl' (swift, quick), which evolved to mean 'high' or 'tall.' The superlative suffix '-est' comes from Old English and is one of English's oldest word-formation patterns.
Interestingly, what counts as 'tall' is culturally relative—the average human height has grown 10 inches in the past 150 years due to better nutrition, meaning 'tallest' is literally a moving target.
Complete word intelligence in one call. Free tier — 50 lookups/day.