Start with the body: a sore throat, a hurt knee, an old dog getting up. Painfully is how an action goes when it hurts to do it, and when anyone watching can see that it hurts. English then lends that feeling out. It lends it to degree — painfully shy, painfully slow, painfully obvious — where nothing hurts but the speaker wants you to feel the strain. And it lends it to effort — painfully acquired, painfully rebuilt — where the cost is in the work, paid a little at a time.
From the word’s history
Painfully comes from painful plus -ly, going back to Middle English peinfully. Painful is pain plus -ful, so the adverb starts life meaning simply 'in a way full of pain' — which is still the literal sense at the centre.
painfully 由 painful 加 -ly 而來，往上追到中古英語的 peinfully；而 painful 又是 pain 加 -ful。所以這個副詞一開始的意思就是「以充滿痛的方式」，也就是今天最字面的那一個用法，其他兩個都是從它借出去的。
Wiktionary entry 'painfully' (source_ref kaikki:3f6f69a190036ae4:1025912330:6868): 'From Middle English peinfully, paynefully, equivalent to painful + -ly.'
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