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.
early 14c., "founder, originator," agent noun from begin. Meaning "novice" is from late 15c. Beginner's luck is from 1849, originally in gambling.
It forms all or part of: abaft; about; alley (n.1) "open passage between buildings;" ambagious; ambassador; ambi-; ambidexterity; ambidextrous; ambience; ambient; ambiguous; ambit; ambition; ambitious; amble; ambulance; ambulant; ambulate; ambulation; ambulatory; amphi-; amphibian; Amphictyonic; amphisbaena; Amphiscians; amphitheater; amphora; amputate; amputation; ancillary; andante; anfractuous; be-; begin; beleaguer; between; bivouac; but; by; circumambulate; embassy; ember-days; funambulist; ombudsman; perambulate; perambulation; preamble; somnambulate; somnambulism; umlaut.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit abhitah "on both sides," abhi "toward, to;" Avestan aibi; Greek amphi "round about;" Latin ambi- "around, round about;" Gaulish ambi-, Old Irish imb- "round about, about;" Old Church Slavonic oba; Lithuanian abu "both;" Old English ymbe, German um "around."