Is

[Samuraisan]

Is " Knock your socks off " a metaphor or what ?

[fiction-factory]

Yes, because you don't literally knock them off

[Samuraisan]

Thanks ff.

[fiction-factory]

Tilly's around- she will know for certain

[Garaman]

An idiom isn't it?

[jno]

I'm not sure it is a metaphor. It doesn't literally remove your footwear, but it doesn't do so metaphorically either. I think it's an imaginary cartoon situation, where you draw someone with one of those little bombs that look like bowling balls with a fuse sticking out, and when it goes off all your clothes are blown off. But that's not really a metaphor, it's comic exaggeration. I think Garaman is right, it's an idiom rather than a comparison with anything else that actually happens.

[Mamyalynne]

I agree, an idiom.

[Togo]

I think it is a mix up that developed from "knock your block off" and "knock spots off". I tend to think of such sayings as figurative Language.

[woofgang]

but those are idioms too....

[Togo]

//One major factor in popularizing the positive meaning of "knock your socks off" was an ad campaign done by Pepsi-Cola in the mid-1960s to promote Mountain Dew, a regional Southern soft drink purchased by Pepsi-Cola that Pepsi hoped to make popular on a national basis.  Mountain Dew originally got its name for a Southern slang term for moonshine whiskey.  So Pepsi naturally decided to launch an ad campaign that played up Mountain Dew as a quintessentially Southern soft drink, but one that could be enjoyed by people from all regions of the United States.  Since the stereotype was that Southern people liked to go barefoot, the ad campaign encouraged soft drink buyers to "get that barefoot feeling" by buying Mountain Dew, which the ads claimed would "knock your socks off."//

Its on the interweb it mu be true. (^_*)

 https://www.quora.com/What-is-the-origin-of-the-phrase-knock-your-socks-off-and-what-does-it-mean