Late Middle English; earliest use found in Reginald Pecock (c1392–?1459), bishop of Chichester and religious author. From unmighty + -ly. Compare earlier mightily.
Definition of unmightily in US English:
unmightily
adverb(ˌ)ʌnˈmʌɪtɪli
rare
Powerlessly, impotently; weakly, ineffectually.
Origin
Late Middle English; earliest use found in Reginald Pecock (c1392–?1459), bishop of Chichester and religious author. From unmighty + -ly. Compare earlier mightily.