Loudly

google

ref

Old English hlūd, of West Germanic origin; related to Dutch luid, German laut, from an Indo-European root meaning ‘hear’, shared by Greek kluein ‘hear’, klutos ‘famous’ and Latin cluere ‘be famous’.

wiktionary

ref

From Middle English loudly, equivalent to loud +‎ -ly.

etymonline

ref

loudly (adv.)

c. 1400, from loud + -ly (2).