beetroot mug, n. — Green’s Dictionary of Slang

beetroot mug n.

[mug n.¹ (1b); ? coined by Charles Ross, creator, c.1867, of the comic character Ally Sloper, a dissipated looking old man with a red and swollen nose; poss. the orig. of SE phr. red as a beetroot]

a red face.

1909

J. Ware Passing Eng. of the Victorian Era.