2. An expression of surprise.
Most examples of "like, whoa" involve quotative be like, as in this gem from 1990:
Nick B. Williams Jr., "Woman's Spirited Escape: 'We Were, Like, Whoa! So Weird' : Refugees: San Fernando Valley native braved tanks and table tennis on trek from Kuwait to Saudi Arabia", L.A. Times 8/29/1990:
The Iraqi army came to Kuwait four days before Michelle Mateljan of Studio City, Calif., was to fly out for a vacation in Spain.
"I was so bummed," she said Tuesday.
Now safe in Bahrain, Mateljan recounted her ordeal, including a final scare at the last Iraqi checkpoint at the Saudi Arabian border.
"There were six or seven soldiers, and we thought, 'This is it,' " the 22-year-old brunette recalled over a plate of pasta. "So what happens? They gave us a can of beans and some chocolate bars. We were, like, whoa! So weird."
But I'm pretty sure that I heard "like whoa man", with the discourse-particle version of like, more than once in the 1960s.
An intermediate step along whoa's journey from horse-command to expression of surprise would have been a metaphorical request to stop or reverse a horse-less action or interaction, e.g.
Sidney Kingsley, "Night Life: In Three Acts" (publication dated 1966, but the play was performed on Broadway in 1962):
I imagine that this usage must have begun pretty much as early as the horse-command did, but a quick search didn't turn up any examples from Melville's time.