Sunder, v. t. [imp. & p. p. Sundered; p. pr. & vb. n. Sundering.] — Webster enters it under “sunder”
Etymology: [OE. sundren, AS. sundrain (in asundrain, gesundrain), from sundor asunder, separately, apart; akin to D. zonder, prep., without, G. sonder separate, as prep., without, sondern but, OHG. suntar separately, Icel. sundr asunder, Sw. & Dan. sönder, Goth. sundro alone, separately.]
To disunite in almost any manner, either by rending, cutting, or breaking; to part; to put or keep apart; to separate; to divide; to sever; as, to sunder a rope; to sunder a limb; to sunder friends. It is sundered from the main land by a sandy plain. Carew.
3 other Webster entries under this spelling
Sunder, v. i.
To part; to separate. [R.] Shak.
Sunder, n.
A separation into parts; a division or severance. In sunder, into parts. "He breaketh the bow, and cutteth the spear in sunder." Ps. xlvi. 9.
Sunder, v. t.
To expose to the sun and wind. [Prov. Eng.] Halliwell.
Shades of meaning
Fernald lists sundered only as an antonym, under mutual.
He lists it without comment; the entries above explain its neighbors.