Used other than figuratively or idiomatically: see por que, não. · why not (used to state one has no objection) · why not (used to introduce a proposal)
why wouldn't one?, what else if not?, how ... not?, i.e. naturally, of course, sure one would
köken: quid (“what, why”) + nī (“unless, if not”), i.e. "what else if not?", "how else?". Compare Russian как же (не) (kak že (ne), “what else (if not)?, of course”) and а как же (a kak že).
Quid egō nī ita cēnseam?
Quid egō nī fleam!
– Omnem rem tenēs? – Quid ego nī teneam? – You understand the whole affair? – What's there to not understand?