why

why [hwī, wī]

adv.

[ME hwi < OE, instrumental case of hwæt, WHAT]

for what reason, cause, or purpose? with what motive?: used in direct, indirect, and implied questions [why did he go? he told her why he went]

conj.

1. because of which; on account of which [he knows of no reason why you shouldn't go]

2. the reason for which [do you know why he went? this is why he went]

n.

pl. whys

the reason, cause, motive, purpose, etc. [never mind the why and wherefore]

interj.

1. used to express surprise, impatience, indignation, etc.

2. used as an expletive, to preface a remark

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why not

Informal all right; it's acceptable: used to express qualified approval of what has been proposed, suggested, etc.