Mightily grows out of might — plain strength. Start with a body putting everything it has into something: against the tide, hauling stones, calling out across a valley. English then lets that same strength leak into feelings, so mightily relieved is relief with a whole body behind it and mightily proud is pride you can hear from across the room. The strength never leaves the word. It just stops being muscle and becomes emphasis.
From the word’s history
The packet's Wiktionary etymology runs Middle English mightili, from Old English meahtiglice, from mihtig ('powerful, mighty') plus the adverb suffix -lice; mihtig goes back to Proto-Germanic *mahtiz, 'force, strength'. The strength in the modern word is the same strength that was there at the start.
證據包裡的字源是：中古英文 mightili ←古英文 meahtiglice ← mihtig（有力量的）加副詞字尾 -līċe；mihtig 再往前接到日耳曼語 *mahtiz（力量、能力）。今天字裡的那股「大」，就是最早那股力氣。
Wiktionary entry supplied in the evidence packet (source_ref kaikki:3f6f69a190036ae4:1026644018), etymology_text field.
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