The titular expression refers to the fact that physical attractiveness is merely a portion of true beauty, which stems from the inside: the soul.
While it is unclear where the expression is originating from, an early documented case of the phrase “Beautiful inside and out” can be found in Joseph Woods’ Letter of an Architect, from France, Italy, and Greece, where the author is describing the church of St. Urban as “Beautiful inside and out”.
The expression grew in the 20th century, used by poets and authors to describe physical beauty and a kind personality with the same phrase, although it started to really take off in the final decades of the 1900’s.
With the rise of the internet, the phrase “Beautiful inside and out” became a cliché, well-liked by women on social media, similar to Live Laugh Love.
“Beautiful inside and out” is most seen in comments, post descriptions as well as inspiring image macros, used to motivate people on the web.