Use it to describe strong emotions that impact one's physical state, not just any feeling.

Emotional Overwhelm

It's a figurative expression; your knees might not actually feel weak.

Non-literal Use

Shows an extreme reaction, more than being just nervous or excited.

Often used when describing a strong romantic attraction or love.

It's metaphorical and speaks to the power of one's attraction, not physical weakness.

Expresses a deep level of affection, more than a casual liking.