If someone has their hair in a topknot, the hair is arranged in a small neat pile on top of their head.
topknot in British English
(ˈtɒpˌnɒt)
noun
1.
a crest, tuft, decorative bow, chignon, etc, on the top of the head
2.
any of several European flatfishes of the genus Zeugopterus and related genera, esp Z. punctatus, which has an oval dark brown body marked with darker blotches: family Bothidae (turbot, etc)
topknot in American English
(ˈtɑpˌnɑt)
noun
1.
a knot of feathers, ribbons, etc. worn as a headdress
2.
a.
a tuft of hair on the crown of the head
b.
a tuft of feathers on a bird's head
Examples of 'topknot' in a sentence
topknot
There was a little pink bow in its topknot, which I transferred mentally to Miss Webster's own head.
Hilary Mantel LEARNING TO TALK: SHORT STORIES (2003)
On her last stagger to the loos, she'd tied her hair up in a topknot, which Joan was now looking at with contempt.
Cathy Kelly JUST BETWEEN US (2003)
Late the next afternoon Molly lowered one hand from the steering wheel and ran her fingers through Roo's topknot.