The strongest, most powerful, or most capable of all.
From Old English 'miht' meaning power or strength, related to 'might' (can, have power). The superlative '-est' makes it the highest degree. The word traces back to Germanic roots meaning 'to have power.'
Interestingly, 'might' and 'main' (as in 'with all one's main') come from the same root word—both originally meant the physical strength of muscles! So when heroes in old stories have 'mighty strength,' they literally have strong main muscles.
Complete word intelligence in one call. Free tier — 50 lookups/day.