1. in a situation or position where you are confident or secure
Professional
Social
I was on solid ground discussing that subject
If you say you are 'on solid ground' it means you are safe or secure usually because you are in a situation where you feel confident or knowledgeable. Literally to 'be on solid ground' means to land safely from a boat or flight onto the ground so when you use this phrase figuratively it means to be safe in your argument, position or abilities.