just took a peek(!) in OED and unray is marked [(Brit) dial] with the def'n as given above. here are the most recent citations:

1599 Hakluyt Voy. II. ii. 57 One of the Spaniards vnraied himselfe, and lept into the water. 1611 Cotgr., Desabiller, to vncloath, vndresse, vnray. 1825– in s.w. dialect glossaries.
absol. 1867 W. F. Rock Jim an' Nell lxxix, Zum.. chap 'll help thee to unray.

(Zum?)

the etymology simply refers to the prefix un-, so:

4. a. A small number of OE. verbs in un- imply removal or deprivation; these end in -ian, as unhádian to deprive of orders, unhlidian to remove the lid from, uninseglian to unseal, unscógian to unshoe. In ME. the type remains rare, but occurs in unclead, unclothe, unhair. At a later date it becomes more frequent, and is common in modern use.

then, following the trail of discarded aritcles, under the verb ray:

4. To dress (oneself or another). = array v. 8. Also absol. Now dial.