Note: The form 'knicker' is used as a modifier.
1.
Knickers are a piece of underwear worn by women and girls which have holes for the legs and elastic around the waist to hold them up. (BRIT; in AM, use panties )
She bought Ann two bras and six pairs of ~.
N-PLURAL: also a pair of N
2.
If someone is getting their ~ in a twist about something, they are getting annoyed or upset about it without good reason. (BRIT HUMOROUS, INFORMAL)
PHRASE: V inflects