A linguistically-adopted MetaSyntacticVariable?
representing a person of unknown or unspecified identity.
It can be used in a sarcastic sense when the person is implicitly (or sometimes explicitly) specified, as in "Looks like SomeBody has a case of the Mondays" [OfficeSpace].
Alternatively, a person of importance.
"Some Body" is also the title of a BubbleGumCrash?
image song (by Yuiko Tusbokura?).