Literally: old – now – nothing – pair
Alternately: Something is unmatched from the past to the present. A quality (often, but not always, greatness) is unequaled at any time in history, right up to the present moment.
Notes: 古今 does not simply mean “past and present,” as one might guess from the characters’ meanings; it refers to the entire span of time. There are a number of four-character compounds beginning with 古今, including several synonyms to this one. (For example 古今無比 (~.hi), in which the character 比, which replaces 双, means “compare.”)