fat in the fire

[ostap77]

"Susie knows you've been seeing her boyfriend, so the fat's in the fire."

My Cambridge dictionary says it's old-fashioned. Do people say this idiom nowadays?

 

[5jj]

I do, but I am often accused of being old-fashioned.:roll:

 

[ostap77]

I do, but I am often accused of being old-fashioned.:roll:

I mean in general.

 

[riquecohen]

I do, but I am often accused of being old-fashioned.:roll:

Same for me. I usually hear it (or say it) in relation to an already unpleasant situation. You have been having problems with Susie and now you are seeing her boyfriend. You've just "thrown more fat into the fire."
Addition: It's used by people I know who are generally over 50. I can't speak for younger people.