"letters of a language arranged in customary order," 1570s, from Late Latin alphabetum (Tertullian), from Greek alphabetos, from alpha + beta. Attested from early 15c. in a sense "learning or lore acquired through reading." Words for it in Old English included stæfræw, literally "row of letters," stæfrof "array of letters," and compare ABC.
It was a wise though a lazy cleric whom Luther mentions in his "Table Talk,"—the monk who, instead of reciting his breviary, used to run over the alphabet and then say, "O my God, take this alphabet, and put it together how you will." [William S. Walsh, "Handy-Book of Literary Curiosities," 1892]
Alphabet soup is attested by 1907.
also a-b-c, late 13c. (spelled abece) from the first three letters of it taken as a word (compare alphabet, abecedary, Old French abecé, abecedé "alphabet," 13c.). Sense "rudiments or fundamentals (of a subject)" is from late 14c. As a shortening of American Broadcasting Company from 1944 (in a Billboard magazine headline), earlier of Australian Broadcasting Corporation (1931). Related: ABCs.
second letter of the Greek alphabet, c. 1300, from Greek, from Hebrew/Phoenician beth (see alphabet); used to designate the second of many things. Beta radiation is from 1899 (Rutherford). Beta particle is attested from 1904. Beta male, pejorative term for a risk-avoidant, non-confrontational man perceived as a follower or supporter rather than a leader, is by 2005, transferred from zoology (birds, primates), where it is attested by 1962 (compare alpha male under alpha).