"to boldly split in finitives that no man had split before"

A split infinitive is a phrase where an adverb is inserted into the infinitive form of a verb, between "to" and the verb. While technically grammatically incorrect, it is commonly used for emphasis. 

The most famous example of a split infinitive, and the one alluded to here, is "to boldly go", from the introductory narration to Star Trek: