A GOBSMACKING ETYMOLOGY

The word gobsmacked (a whimsical way to say "astonished") was first written down in a 1935 edition of a periodical called the Beekeeper's Record, in the following sentence:

When he landed back Martha wad be fare gob smacked at the yarns he wad tell 'er about Yorkshire clod-hoppers.

This near-incomprehensible quote is a great example of the very colloquial history of the word. It combines our familiar term smack with the Scottish Gaelic noun gob, which meant "mouth" (the idea was that if something was gobsmacking, it was akin to getting hit in the mouth). That percolated through various northern English dialects, becoming especially associated with Liverpool, and was then popularized through television programmes set in the area. The etymology of gob is uncertain; it could be related to the Middle English verb gobben, which meant "drink greedily", or another Gaelic word, which was also spelled gob and meant "beak". Smack​ is just imitative of the sound of a strike.