My apologies for the length of this, but the OED has it pretty clearly:
[f. OF. a(s)soille pres. subj., a(s)soil pres. indic. of a(s)soldre, a(s)soudre:L. absolre = absolvre to absolve, f. ab from + solvre to loose. Other forms of the infinitive in OF. (the first two also in AF.) were a(s)soilier, a(s)soiler, a(s)solier, a(s)soillir, a(s)solir, as if:L. *absolure. L. solre gave OF. solre, soldre, as batre, quatr gave batre, quatre, and volurunt, *volrunt gave volrent, voldrent; with the variants a(s)soilier, a(s)oillir, compare other OF. double forms, as tesir, taire:L. tacre, *tacre, and plesir, plaire:L. placre, *placre. Subsequently refashioned in Fr. as absoudre, and in Eng. as ABSOIL, which paved the way for the modern ABSOLVE, formed directly from the L. after 1500. The Fr. l mouillé, lost in Eng., was as usual retained in Scotch, and symbolized by l, lyh, ly, now corruptly written lz, whence the current assoilzie (solj, sol).]
I. To assoil a person.
1. To absolve from sin, grant absolution to, pardon, forgive; = ABSOLVE 2. ‘Whom God assoil!’ (OF. que Dieu assoille! L. quem Deus absolvat!): an ejaculatory prayer for the departed. arch.
1297 R. GLOUC. 464 No man, bote e pope one, hem asoyly ne myte. 1340 Ayenb. 172 et he habbe power him to asoyli and him penonce to anioyni. c1340 Gaw. & Gr. Knt. 1882 Of absolucioun he on e segge calles, & he asoyled hym. 1426 Pol. Poems II. 131 As wele on his ffader side, Henry the fifth, whom God assoille, as by Kateryne quene of Englond, his modir, whom God assoile. 1610 HOLLAND Camden’s Brit. I. 564 Pray devoutly for the soule whom God assoile. 1638 Penit. Conf. vii. (1657) 132 God remitting whomsoever the Priest assoileth. 1816 SCOTT Antiq. xxvi, ‘God assoilzie her!’ ejaculated old Elspeth..‘His mercy is infinite.’ c1840 DE QUINCEY Autobiog. Sk. Wks. II. 102 Oxford might avail to assoil me.