Green’s Dictionary of Slang

excellent, very good; often with at/on/to (i.e. very good at something).

1912		E. Pugh City Of The World 238: I don’t mind sums, though. I’m mustard at them.
1927		E. Wallace Squeaker (1950) 8: Is there any chance of seeing this Barrabal I hear so much about? They say he’s mustard.
1947		J. Maclaren-Ross Of Love And Hunger 135: Make no mistake, she’s mustard.
1954		Oh Boy! No. 21 10: Although he always gets into a pickle, you must admit he’s mustard.
1978		(ref. to 1917–18) H. Berry Make the Kaiser Dance 101: In World War I parlance, a man who was on the square was the real goods — in other words, a right guy [...] just let it suffice to say it meant he was ‘to the mustard’.
1993		(con. 1945) S. McAughtry Touch and Go 57: I didn’t know you could fight like that [...] My God, you’re mustard.
2005		B. Hare Urban Grimshaw 168: They did it in less than a week, which was mustard.
2021		P Howard Braywatch 32: ‘I didn’t mean to piss on your parade.’ He’s like, ‘Ine moostard, Rosser – doatunt woody abourrit’.