It can be used as an adverb, a noun, or even a preposition.
Preposition: With the exception of; except; save:
No one replied but me.
Noun: Buts, reservations or objections:
You'll do as you're told, no buts about it.
Adverb: Only; just:
There is but one God.
This three letter word holds the power to change everything and lie to ourselves.
Trump campaign is raising cash. How's it spending it? http://nbcnews.to/2bJYznC via @nbcnews
A 3-Letter Word That Can Make You Look Bad @MarcelSchwantes http://www.inc.com/marcel-schwantes/a-3-letter-word-that-can-make-you-l… via @Inc
Indian actor’s creepy ‘real ghost’ encounter goes viral - but what was it? (VIDEO, POLL) https://www.rt.com/viral/356609-india-actor-ghost-video/#.V7ijUwcsvs4.t…
The International 2016: the greatest event not just in Dota 2 but in all of e-sports http://arstechnica.com/?post_type=post&p=941603
Keep it short and simple... We've all Kissed BUT.