It is teatime
Name ref, Archibald is an English name and teatime is an English custom
Child of masked ashes
Reference to Mandrake, his mother, who is gray with a white mask
Wear your feathered headdress if you must
Title ref, Native American chiefs often wore feathed headresses
Streak of absence defacing all the colors
Refers to his blaze. White is defined as the absence of color, while black is a combination of all colors. Defacing was a clue to the fact it is a facial marking
No time to waste
Fluff
Eat your cake and let us be on our way
Fluff
We're late, my giant friend
Size ref, Archibald is over 18 hands
The thread title, Impossible
It would be nearly impossible for Chester, who is only 7 hands, to subdue Archibald and hold him captive for a week.