When someone says to you "your mustard you"

[JoanneBo]

When someone says to you "your mustard you"?

[Xollob]

I remember phrases such as "you're mustard, you", or "she can be right mustard, she can" from my childhood in Lancashire in the 60s and 70s. It means something like stubborn, difficult, trying, or pig-headed. I haven't heard it for many, many years and assume it is obsolete, dialect, or both.

[Dusky]

I wonder if it's "you're mustard, you" rather than "your mustard you" and grown from the phrase "keen as mustard".  It sounds like a sardonic retort to someone who apparently only just saw the point after everyone else, as in "Oh, you're quick!"