1520s, "of or pertaining to a beginning," from Middle French initial or directly from Latin initialis "initial, incipient," from initium "a beginning, an entrance," from past participle stem of inire "to go into, enter upon, begin," from in- "into, in" (see in-(2)) + ire "to go" (see ion).

"initial letter of a name or surname," 1620s, from initial, adj. in a specialized sense "standing at the beginning of a word, sentence, etc."

"to mark or sign with initials," 1864, American English, from initial, n.. Related: Initialed; initialing.