"Square peg in a round hole" is an idiom describing the unusual (?) individualist who could not fit into a niche of their society.
The metaphor was originated by Sydney Smith in "On the Conduct of the Understanding", one of a series of lectures on moral philosophy that he delivered at the Royal Institution in 1804–06:
"If you choose to represent the various parts in life by holes upon a table, of different shapes,—some circular, some triangular, some square, some oblong,—and the person acting these parts by bits of wood of similar shapes, we shall generally find that the triangular person has got into the square hole, the oblong into the triangular, and a square person has squeezed himself into the round hole. The officer and the office, the doer and the thing done, seldom fit so exactly, that we can say they were almost made for each other."
- https://en.wikipedia.org/wiki/Square_peg_in_a_round_hole with editing
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Noticed I put "?" in wiki's description above?
IMHO, no human is in perfect shape, be it round or square. Everyone and each of us are on the spectrum, so "unusual" is a relative term.
I myself am an "unusual" shaped "peg" who never felt very fit in any niche, even when I wanted to.
Eg. I started asking myself question when I was 6 years old: "Who am I and why am I in this body?" (still don't know the answer now 
); I never liked to compete with others in schools/Uni and the like; I prefer simple life over the rat-race that I left work at the peak of career; My wechat “朋友圈” has only a handful whom I know for decades and who can torelant my frankness; I felt like a piece of 出土文物 when I first came to WXC 
... ...
Hey, being a "square peg in a round hole" or not, I'm still glad to be a human with all parts intact. 
Happy Friday Everyone!
