To be physically capable of going into a space(verb)
To be confident in a social situation(verb)
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Use "fit in" in a sentence
"“Neil fit in perfectly,” Mechem declares, “because there was nobody quite like Neil.”"
"Such a theory does not fit in with the content of these psalms; they are not temple-psalms."
"HAGERSTOWN - Clara Hamilton was ahead of her time, with her plan for what is now known as the Oak Hill Historic District in Hagerstown's North End. Even though she lived from 1837 to 1919, her ideas would fit in with the current interest in home gardens, green technology and healthier lifestyles."