Holy smoke! Holy cow! Holy toledo! Holy moley! Holy mackerel! are all exclamations or slight expletives. In general, they have passed out of everyday usage, except in the vocabulary of elderly people or older TV shows.

It's exactly like Oh my God! It's a bit dated so you'll probably hear oh my god more often than holy smokes even though some people do still say it. Here's an example: "Holy smokes! It's 9 already?! I'm late!" Or "$800? Holy smokes, that's expensive!!"

I think Robin (Batman's sidekick) used to say that. There was a joke in one of the Batman movies where he would say variations on the prase.

Dina's explanation is a good one. I would add that you almost never hear "holy smokes" any more. It has almost passed out of usage.

Holy Smokes is an exclamation of surprise and annoyance. The synonyms are Good God!, Good Gash!, Good Glasious! I hope it will help.