"Reasonable excuse" refers to having a valid and acceptable reason for not doing something or for acting differently than expected. It means having a logical and understandable justification for your actions or lack thereof. Full definition
There are some bad behaviors that are so commonplace and easy to fall into, we've come up with seemingly reasonable excuses for them. (entrepreneur.com)
He was charged with failing to give a sample without reasonable excuse. (labour-uncut.co.uk)
Good old - fashioned fear has disguised itself as a perfectly reasonable excuse not to do something that might just be your life's purpose. (mindbodygreen.com)