Green’s Dictionary of Slang

an eccentric.

1756		W. Cowper in Connoisseur 16 Sept. 833: Humbuggers, and those who ‘nickname God's creatures,’ and call a man a cabbage, a crab, a queer cub, an odd fish, and an unaccountable muskin, should never come into company without an interpreter .
1922		Joyce Ulysses 383: So many agreeable females with rich jointures [...] lose their womanly bloom in the embrace of some unaccountable muskin.