Green’s Dictionary of Slang

(a) (US) spirit, zest, courage, esp. in adversity.

1614		Jonson Bartholomew Fair I ii: Not a corn of true salt nor a grain of right mustard amongst them all.
1903		A. Adams Log of a Cowboy 237: They [i.e. a couple of dogs] were not the proper mustard.
1977		Eble Campus Sl. Apr. 3: mustard – power, speed: Put some mustard in it!

(b) one who is keen or the best, outstanding, excellent at a task or occupation; the best; often in phr. all to the mustard, proper mustard, the genuine article.

1903		A. Adams Log of a Cowboy 237: And for fear they were not the proper mustard, he had that dog man sue him in court for the balance, so as to make him prove the pedigree.
1906		Goodwin’s Wkly (Salt Lake City, UT) 18 Aug. 7/2: I’ve got the folks up here thinking I’m all to the mustard.
1910		‘O. Henry’ ‘Strictly Business’ in Strictly Business (1915) 5: Bob Hart, ‘All the Mustard and a Whole Show in Himself’.
1912		Day Book (Chicago) 5 Mar. 15/1: One of them ginks that thinks he is all to the mustard with the dames.
1916		Somme-Times 31 July (2006) 118/2: Tres moutarde, if you understand her, / Is Cousin Blanche of ‘The Bystander.’.
1943		Tennessean (Nashville, TN) 27 Dec. 2/3: He praises a smart pilot as a ‘Mustard’.

(c) a woman who is seen as sexually enthusiastic; something, e.g. a play, that is sexually overt [play on hot stuff n.² (3); note comment, in T.R.G. Lyell, Slang, Phrase & Idiom (1931) ‘It must never be used of the female sex’].

c.1908		Harrington & Powell [perf. Marie Lloyd] The Directoire Girl 🎵 The Premier looked askance, and then had another glance / I heard Winston Churchill mutter ‘Mustard’.
1923		‘Bartimeus’ ‘Chops and Chips’ in Seaways 108: ‘Très moutarde – eh?’ ventured ’Orace.