They laughed because Kickers is a word for women's underwear!
In England we also call them pants, but be careful because pants means trousers in North American English.
Kickers, pants, underwear - all these words refer to the same thing in British English: female underwear (the bottom half).
Stroppy knickers could mean becoming unecessarily upset and being moody. It means stroppy.
We have another idiom in England which involves the words knickers:
- For Teachers