1. a field that does not suit someone's abilities
that's not where my strength lies, I'm good at other things
'Where your strength lies' is a way of saying what your talents, skills and aptitude are. If you say 'that's not where my strength lies' you mean something is outside your area of expertise or outside your skillset and you are not confident about being able to do it well. This is a polite way of saying you have no experience in something you are being asked to do.