This expression, which almost always follows a verb, is a reduction of ". . . of ten [points]" ("unbeatable, with the highest possible rating"). This is compared with the score from 0 to 10 (minimum to maximum) used, among other cases, for school evaluation. The addition of the preposition "de" doesn't seem to fit, but there are already cases in Spanish where it is used, as in "de lo mejor".
