The story
From bold (adjective) + em- -en (variant of en- -en (circumfix forming transitive verbs denoting an increase in, or intensification of, the quality denoted by the enclosed word)). Compare Middle English embolden, enbolden (“to make bold, encourage”) (modern English embold, enbold, imbold (obsolete)), and inboldysshen (“to embolden, encourage”).
Etymology from Wiktionary, CC BY-SA 4.0.