[OE. understondan, -standan (under-1 8 a), = OFris. understonda, MDa. understande, MSw. undi(r)standa, OIcel. (as a foreign word) undirstanda. Cf. MLG. understân to understand, to step under, MDutch onderstaen (Dutch -staan), MHG. understân, -stên (G. unterstehen), to take upon oneself, to venture, presume, etc.
With a different prefix, the same use of stand appears in OE. forstandan, OS. farstandan, OHG. far-, firstantan (firstân), and MHG. verstân, -stên (G. verstehen), MDutch verstaen (Dutch -staan). In the 15th and 16th cents. three forms of the past participle were current, viz. (a) the original understanden (also -stonden), in use till about 1550; (b) the reduced form of this, understande (-stonde), -stand (-stond), common till about 1575, and surviving into the 17th cent.; (c) the new form understanded (-stonded), very common from about 1530 to 1585. The occurrence of understanded in the Thirty-Nine Articles, xxxv, in the phrase `understanded of the people', has given rise to recent echoes of it, especially in journalistic use. The modern form understood came into use in the latter part of the 16th cent., and was usual by 1600. ]
Best Answer
People have already said that suf- is a form of sub- (it can also be found as suc-, sug-, sum-, sup-, sur- and sus-).
What they left out is that it's opposite is super- or sur-. Yes, that does mean that one of the forms of sub- is exactly the same as one of the forms of its opposite! Languages that form organically over thousands of years aren't always logical.
A lot of cases with sub- and its variants have no logical opposite though, since e.g. subcutaneous means below the skin, but "above the skin" makes no sense, since we don't have any bit of us above the skin.
The example you give though, is an exception again. As suffix's antonym is prefix, because they don't have perfectly corresponding component parts. Again, English isn't always logical.