What does it mean?
Everyone has private problems you can't see, so treat people gently. The original 1897 wording was "Be pitiful, for every man is fighting a hard battle."
Who said it?
Ian Maclaren: Scottish author and minister (1850–1907).
These exact words are widely credited to Ian Maclaren, but they don’t appear in their known work. They may be a later paraphrase or a misattribution.