What? What did you say?

what / that thing (used in place of words you can't remember at the moment.)

Nothing

What?, Huh?

(adverbial particle, dismissive, derogatory) emphasizes disgust, contempt, or otherwise negative feelings of the speaker: whatever

何: interrogative pronoun; what

何か: something

Used other than figuratively or idiomatically: see なに, これ.

何者: someone; anyone / 何物: what

whatchamacallit, X