Nu (ぬ) is an auxiliary verb suffix in Japanese that turns a verb into a negative verb. It's similar to the -nai suffix that most beginning Japanese students learn, but it's much older and more rarely used.

-Nu is appended to the same base as -nai, but conjugates differently. In the renyokei form (-naku), it becomes -zu. In the conditional form (-nakere), it becomes -ne. If that made no sense at all to you, here are some examples on how -nu is used:

Dôshite mo ikaneba naranu (= Dôshite mo ikanakereba naranai).
I have to go no matter what.

Ikenu mono wa dô shiyô ka? (= Ikenai mono wa dô shiyô ka?)
What shall we do about those who cannot go?

Shirazu aruite kita (= Shiranaku aruite kita).
In spite of myself, I walked over.

The final -n in polite -masen verbs is actually a contraction of -nu.