why not

Short definition from Wiktionary

Adverb

1. 1
   Why is that so? (for a negative statement)
2. 2
   Used to state that one has no objection to and approves of a proposal.
3. 3
   Used to introduce a proposal.

Noun

1. 1
   A violent and peremptory procedure without any assigned reason; a sudden conclusive happening.