Probably a corruption of Abron, meaning auburn. Halliwell quotes the following from Coriolanus, ii. 3: “Our heads are some brown, some black, some Abram, some bald.” And again, “Where is the eldest son of Priam, the Abram-coloured Trojan?” “A goodly, long, thick Abram-coloured beard.” —Blurt, Master Constable.
Hall, in his Satires, iii. 5, uses abron for auburn. “A lusty courtier ... with abron locks was fairly furnishëd.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894