1520s, "of or pertaining to a beginning," from Middle French initial or directly from Latin initialis "initial, incipient, of the beginning," from initium "a beginning, a commencement; an entrance, a going in," noun use of neuter past participle of inire "to go into, enter upon, begin," from in- "into, in" (from PIE root *en "in") + ire "to go" (from PIE root *ei- "to go"). Related: Initially.
"to mark or sign with initials," 1837, from initial (n.). Related: Initialed; initialing.
"initial letter of a name or surname," 1620s, from initial (adj.) in a specialized sense "standing at the beginning of a word, sentence, etc." (1620s).