Old English beginnan "to attempt, undertake," a rare word beside the more usual form onginnan (class III strong verb; past tense ongann, past participle ongunnen); from be- + West Germanic *ginnan, which is of obscure etymology and found only in compounds, perhaps "to open, open up" (compare Old High German in-ginnan "to cut open, open up," also "begin, undertake"), with sense evolution from "open" to "begin." Cognates elsewhere in Germanic include Old Frisian biginna "to begin," Middle Dutch beghinnen, Old High German beginnan, German beginnen, Old Frisian bijenna "to begin," Gothic duginnan.
From late 12c. as "originate, be the originator of;" from c. 1200 as "take the first step in, start to deal with." Intransitive sense "come into existence" is from mid-13c.