whats the meaning of (If you fancy the pants off her as well, she's a 24-carat keeper)?

Question

Answer

Fancy the pants off means someone is attracted physically by another one.

Answer

To fancy - to charm someone, make them like you
To fancy the pants off her - to charm her so much, that she will have sex with you
24 carat (gold) - the best quality {the purity of gold is measured in carats with 24 carat gold being 100% pure, but too soft to use for most things other than gold bricks}
Keeper - anything or anyone that is good enough to keep

If you happen to charm her so much that she sleeps with you, she is a really fantastic person that you should stay with, unlike all of those other women you have fancied the pants off of.