An Explanation

Says Blue-and-Buff, to Drab-and-Pink,
"I've heard the hardest word I think,
That ever posed me since my teens,
I wonder what As-best-os means!"

Says Drab-and-Pink, to Blue-and-Buff,
"The word is clear, and plain enough.
It means a Nag wot goes the pace,
And so as best os wins the race."