I don't recall which of the many print books I read in my gearhead/TIG-welding days had used the term, but the lack of a distinct fatigue/endurance limit [0] for these metals was put into practical terms with something along the lines of "like slightly bending a cold stick of butter".

It's why aluminum airplanes are supposed to get inspected for cracks regularly, and why aluminum bicycle frames must be so uncomfortably rigid. If there's cyclic stress occurring, it's a question of when, not if they will crack.

If I had to guess it's from Carroll Smith's 80s-era Engineer to Win, it sounds like something he'd write.

[0] https://en.wikipedia.org/wiki/File:S-N_curves.PNG

I think it refers to the type of break the joint makes?

However, while searching I found this search excerpt:

    Which metals diffuse into other metals and how? | Naked ...
    www.thenakedscientists.com › forum
        Sodium and potassium metal (both *soft solids, like cold butter*) react

So perhaps it is used to designate 'soft solids' versus 'hard solids'