Green’s Dictionary of Slang

(Irish) to comb.

1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor III 31/1: It all sticked to my hair, and I couldn’t rack (comb) right for some time.
1893		Somerville & Ross Through Connemara in a Governess-Cart n.p.: The mermaid, when not decoying sailors to their fate, is incessantly ‘racking her poll’ as they say in the County Cork [BS].
1913		A. Irvine My Lady of the Chimney Corner 81: it was finally decided that I should face the ordeal. My preparation was to wash my feet, rake my hair into order, and soap it down.
1918		K.F. Purdon Dinny on the Doorstep 88: When you’ll have your face washed and your head racked, you’ll be fit to go see the Queen!
1985		M. Johnston Around the Banks of Pimlico n.p.: After washing and dressing herself she would let down her hair and ‘rack’ it with a big comb [BS].
1991		J.B. Keane Love Bites and Other Stories 77: He received a rough and ready shave from the corpse dresser and his hair might have been racked as the saying goes.