whoa interj.

Old English hwa "who," sometimes "what; anyone, someone; each; whosoever," from Proto-Germanic *hwas (source also of Old Saxon hwe, Danish hvo, Swedish vem, Old Frisian hwa, Dutch wie, Old High German hwer, German wer, Gothic hvo (fem.) "who"), from PIE root *kwo-, stem of relative and interrogative pronouns.

exclamation of surprise, etc., c. 1300; as an exclamation calling attention or demanding silence, late 14c. Used after the name of a place to which attention is called (as in Westward-Ho) it dates from 1590s, originally a cry of boatmen, etc., announcing departures for a particular destination. Ho-ho-ho expressing laughter is recorded from mid-12c.

1510s, Scottish, a natural expression of amazement. "This old interjection had a new popularity in the early 1900s and again during the 1960s and later" [DAS].