Green’s Dictionary of Slang

1. of weather, hot.

1997

R. Puxley Fresh Rabbit.

2. angry, hot adj. (5a)

1936		J. Curtis Gilt Kid 242: These Yiddisher Kids were mustard. They hated like hell to think that there was any dough that hadn’t come their way.
1938		J. Curtis They Drive by Night 26: I left me barrer outside. Copped out twice last week for obstruction. Bleeding mustard they’re getting now. [Ibid.] 42: He’s bloody mustard against giving lifts. [Ibid.] 56: The police were getting mustard about lorry gels now.