Dyed in the wool. A hearty good-fellow. Cloth which is wool-dyed (not piece-dyed), is true throughout “and will wash.”
Maister Mainwaring's much abuzed, Most grievously for things accuse, And all the dowlish [devilish] pack; E'en let mun all their poison spit, My lord, there is no wooll zo whit That dyers can't make black.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894