en_tn_condensed/mrk/14/68.md

denied

This means to claim that something is not true. In this case, Peter was saying that what the servant girl said about him was not true.

neither know nor understand what you are talking about

Both "know" and "understand" have the same meaning here. The meaning is repeated to add emphasis to what Peter is saying. Alternate translation: "I really do not understand what you are talking about" (See: rc://en/ta/man/jit/figs-doublet)